Advances in Electrocatalytic Hydrogen Sulfide Splitting for Sulfur Recovery: From Reaction Mechanisms to Application

Abstract

Hydrogen sulfide (H₂S), a highly toxic gas, is mainly sourced from petroleum refining, natural gas purification, and coal chemical processes. It poses significant risks to human health, causes environmental pollution, and accelerates equipment corrosion. Recent studies have demonstrated that electrochemical coupling systems offer an efficient, sustainable, and cost-effective strategy for removing sulfur-containing gaseous pollutants. These systems enable the conversion of H₂S into recoverable sulfur under mild conditions, while simultaneously harnessing the chemical energy of H₂S to drive the production of higher-value products (H₂, HCOOH, CH₄, CO, H₂O₂, etc.). Therefore, electrochemical systems for sulfur recovery have received increasing attention. This review highlights the significance of electrochemical recovery of sulfur from H₂S. It summarizes the reaction pathways and mechanisms involved in anodic sulfur oxidation, critically analyzes and discusses methods for detecting sulfur oxidation products, and summarizes the latest advances in sulfur oxidation reaction (SOR) anode materials and various electrochemical coupling systems. The aim is to enhance the fundamental understanding of electrochemical sulfur recovery and to provide insights for the design of novel SOR electrodes and integrated electrochemical coupling systems.

1. Introduction

With the sustained global growth of global energy demand, the extraction and processing of fossil fuels (oil, natural gas, and coal) and the development of biomass energy (biogas) have expanded rapidly, leading to massive emissions of hydrogen sulfide (H₂S) [1,2,3]. Statistics show that the global annual H₂S emissions from industries such as petroleum refining, natural gas purification, and coal chemical engineering exceed 30 million tons [3,4], presenting a severe environmental burden. Notably, in the purification process of certain high-sulfur natural gas reserves, the volume fraction of H₂S can even surpass 40% [5]. H₂S poses multifaceted and severe threats to human health, industrial safety, and ecological stability: inhalation of air with H₂S concentrations exceeding 300 ppb can cause immediate respiratory arrest [6]; its inherent corrosivity degrades pipelines and equipment, substantially increasing maintenance costs for sulfur-containing gas transportation infrastructure [7,8,9]; furthermore, its release into the atmosphere participates in photochemical reactions, exacerbating acid rain and smog events, which in turn disrupt terrestrial and aquatic ecosystems [10].

As a globally strategic resource, sulfur is indispensable to the chemical, agriculture, and pharmaceutical industries, with a global annual demand of over 70 million tons [4,11]. Traditional H₂S treatment technologies, typified by the Claus process, achieve sulfur recovery but suffer from inherent limitations: severe harsh operating conditions (800–1400 °C), inevitable SO₂ by-products, and low efficiency for low-concentration H₂S (<5 vol%) [12]. In recent years, electrocatalytic oxidation technology has emerged as a promising alternative for H₂S decomposition and sulfur recovery, featuring mild conditions (ambient temperature and pressure), high sulfur selectivity, and reduced energy consumption [2,3]. Under an external electric field, H₂S is oxidized to sulfur at the anode, while value-added reduction reactions (hydrogen evolution, CO₂-to-CO conversion) occur at the cathode, realizing the integration of “waste detoxification-sulfur recovery-energy/chemical production” [13,14].

Owing to the unique advantages of electrocatalytic sulfur recovery technology in H₂S treatment—including mild reaction conditions, high sulfur selectivity, and the capability to couple with the production of high-value products (e.g., H₂, CO)-research in this field has grown increasingly abundant, now covering catalyst design, reaction mechanism analysis, and practical wastewater treatment applications [3,11]. Conducting a thematic review here holds great academic and practical significance to systematically organize the research context, clarify core breakthroughs and unresolved issues, and provide clear exploration guidelines for subsequent researchers. However, relevant reviews remain relatively scarce, and existing research summaries generally lack a systematic elaboration of the “reaction mechanism-catalytic materials-oxidation products-coupling systems” framework: for instance, Yu et al. conducted in-depth analyses of the core mechanism of sulfide oxidation reaction (SOR, e.g., the stepwise oxidation pathway of S²⁻→S_n²⁻→S₈) and electrode material design, and clarified the effect of d-band center modulation on sulfur adsorption/desorption equilibrium via density functional theory (DFT) calculations, but failed to systematically elaborate on the formation principles and detection methods of sulfur oxidation products and completely omitted research on emerging coupling systems such as SOR-CO₂RR [3]; additionally, Shao et al. carried out extensive research on coupling systems for electrochemical sulfur oxidation, optimized operating parameters for practical wastewater treatment, and verified the technology’s applicability, yet they lacked systematic analysis of the catalytic process on the electrode surface and failed to establish a direct correlation between catalytic material structures (e.g., defects, crystal facets) and catalytic performance (e.g., overpotential, selectivity), making it difficult to support the targeted design of subsequent catalysts [3,11,12].

This review summarizes progress in electrocatalytic H₂S splitting for sulfur recovery, structured by the logic chain “reaction mechanism→key components (electrode materials, reactor systems)→product analysis→technical significance”—a framework addressing gaps in existing literature. Unlike prior fragmented reviews, it broadens scope by integrating emerging electrochemical coupling systems and tackling mass transfer limitations in low-concentration H₂S, a key industrial issue long neglected; deepens mechanistic analysis by linking S_n²⁻ polymerization kinetics and S₈ desorption energy barriers to solutions for electrode passivation, beyond superficial pathway descriptions; and sharpens focus on catalysts/processes by classifying electrodes by function (e.g., high-efficiency S₈ production, controllable S_n²⁻ generation) and clarifying structure-performance relationships, moving past unstructured material lists. It further identifies core challenges (sulfur-induced passivation, dilute H₂S mass transfer) and proposes optimization directions, aiming to bridge fundamental research and industrial application.

2. The Significance of Electrochemical Recovery of Sulfur

2.1. Environmental Significance

H₂S is widely recognized as an atmospheric and water pollutant, and its harm to the environment is characterized by multi-dimensionality and long-term effects [15,16]. Given its strong toxicity and widespread emissions, H₂S poses severe threats to ecological stability: In the atmosphere, H₂S readily reacts with oxygen, ozone, and other substances to form SO₂, sulfate, and other compounds [17]. Among these products, SO₂ is one of the primary precursors of acid rain, contributing to soil acidification, forest degradation, and eutrophication of water bodies—ultimately disrupting the ecological balance [18]. Sulfate aerosols, on the other hand, reduce atmospheric visibility and exacerbate smog pollution [19,20]. In aquatic environments, H₂S dissolves to form hydrosulfuric acid, which lowers the pH of water bodies, inhibits the respiratory function of aquatic organisms, causes mass mortality of fish, algae, and other biota, and thus disrupts aquatic ecosystems [21]. Additionally, H₂S can infiltrate and contaminate soil, impairing crop growth and reducing the quality of agricultural products [22,23].

Electrochemical sulfur recovery technology, relying on electrocatalytic oxidation, converts sulfides in H₂S directly into sulfur while avoiding the generation of secondary pollutants such as SO₂ that are inherent in traditional treatment methods (e.g., the Claus process) [1,24]. Moreover, this technology operates under mild ambient temperature and pressure conditions, eliminating the requirement for high temperatures or pressures. This not only significantly reduces process energy consumption and carbon emissions but also aligns with the “carbon peak and carbon neutrality” goals. For industrial wastewater containing H₂S, this electrochemical sulfur recovery technology can further enable in situ treatment, avoiding the risk of leakage during wastewater transportation and thereby reducing pollution to aquatic environments. Therefore, electrochemical sulfur recovery technology holds great practical significance in controlling H₂S pollution and protecting the ecological environment.

2.2. Economic Benefits

From the perspective of resource utilization, the sulfur element in H₂S is a valuable chemical raw material, and the recovery of sulfur can bring significant economic benefits [1,25,26]. Sulfur, as a crucial basic chemical product, is widely applied in the production of sulfuric acid, fertilizers (such as superphosphate and ammonium sulfate), rubber vulcanizing agents, and pharmaceutical intermediates [27]. The global demand for sulfur is stable, and the global sulfur price remained at USD 150–200 per ton in 2024, as reported by IndexBox. Moreover, with the rapid development of the energy industry, the demand for sulfuric acid in the production of lithium battery materials is increasing, further boosting the market value of sulfur [28].

Traditional H₂S treatment technologies, typified by the Claus process, can recover sulfur but suffer from high equipment investment, large land occupation, elevated operating costs, and poor economic efficiency when treating low-concentration H₂S (volume fraction < 5 vol%). Consequently, substantial amounts of low-concentration H₂S are directly discharged or incinerated without sulfur recovery, resulting in the waste of sulfur resources. In contrast, electrochemical sulfur recovery technology features compact equipment, simple operation, and strong adaptability to H₂S concentrations [26]. It can not only handle high-concentration H₂S but also efficiently recover sulfur from low-concentration H₂S, greatly improving the utilization rate of sulfur resources. Additionally, the sulfide oxidation reaction (SOR) as the core anodic reaction at the anode can be coupled with specific reduction reactions at the cathode of the electrochemical system, namely the hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO₂RR). This coupling achieves the synergistic benefits of “sulfur recovery + energy/chemical production”, thereby further enhancing the economic value of the technology [11,29].

3. SOR Mechanism

In the SOR, the oxidation products of S²⁻ include various short-chain polysulfides (S_n²⁻, 2 ≤ n ≤ 6), S₂O₃²⁻, SO₃²⁻, SO₄²⁻, etc.) [3,13,30]. Depending on the subsequent reaction pathways of the SOR intermediate products and the route of sulfur recovery, the SOR pathways can be classified into two types (Figure 1): (1) direct oxidation for sulfur recovery; (2) indirect oxidation for sulfur recovery [31,32]. SOR for sulfur recovery is achieved by successive oxidation reactions of S²⁻ [31]. S²⁻ is first gradually oxidized to form S_n²⁻, and then S_n²⁻ is further oxidized to eventually generate the high-value target product S₈ [29,32]. The entire reaction does not require the introduction of additional media, and the recovery of S₈ can be accomplished solely by successive oxidation steps [29]. Indirect oxidation for sulfur recovery differs from the direct oxidation pathway. This process requires the use of external oxidants (such as Fe³⁺, I₃⁻, EDTA-Fe³⁺, etc.) or reductants (such as S²⁻, S₂O₃²⁻, etc.) as reaction media [29,33]. Through the oxidation/reduction reactions between the media and S²⁻ or SOR intermediate products, the intermediate products are converted into sulfur. The core of this process is to rely on exogenous active substances to drive the conversion of S²⁻ or SOR intermediate products into S₈.

3.1. Direct Oxidation Pathway

In the direct oxidation pathway, sulfide directly adsorbs onto the active sites of the anode surface and transfers electrons directly to the electrode to undergo oxidation reactions and form sulfur [7,10,34]. The direct oxidation pathway follows a stepwise oxidation logic of “S²⁻→S_n²⁻→sulfur” (* + S²⁻→*S²⁻→*S₂²⁻→*S₃²⁻→*S₄²⁻→*S₈²⁻→* + S₈, * denotes the active sites on the catalyst surface), with S_n²⁻ as the main intermediate product, and the generation and transformation of each intermediate have clear potential dependencies [35]. The process can include three key reactions. (1) S²⁻ is gradually oxidized to form S_n²⁻. Under alkaline conditions, HS⁻/S²⁻ first adsorbs onto the active sites of the anode and gradually polymerizes to S_n²⁻ through single-electron or multi-electron transfer. For example, Xiao et al.’s research shows that on the surface of Co₃S₄ nanowire catalysts, S²⁻ is first oxidized to S₂²⁻ (adsorption energy −0.8 to −1.2 eV) and then further converted to intermediate products such as S₃²⁻ and S₄²⁻, demonstrating the dynamic generation process of S_n²⁻ [2]. (2) S_n²⁻ is oxidized to form sulfur. S_n²⁻ is the key intermediate in the direct oxidation pathway, and its further oxidation is the rate-controlling step of the reaction. Density functional theory (DFT) calculations show that on the Co₃S₄ (100) crystal plane, the energy barrier for the oxidation of S₃²⁻ to S₄²⁻ is only 1.40 eV, much lower than 1.65 eV on the (110) crystal plane and 1.85 eV on the (311) crystal plane. This low energy barrier ensures the efficient conversion of S_n²⁻; when the potential rises from 0.4 to 0.6 V (vs RHE), S₄²⁻ and other intermediates further polymerize through S-S bonds and eventually form S₈ [2]. (3) In addition, the desorption ability of sulfur is the core bottleneck of the direct oxidation pathway. High desorption ability of sulfur can lead to its further oxidation to by-products such as S₂O₃²⁻, SO₃²⁻, and SO₄²⁻, resulting in excessive oxidation of sulfur, or sulfur adsorbing on the electrode surface and causing electrode passivation [3,30]. The adsorption energy of S₈ on the Co₃S₄ (311) crystal plane is only 2.52 eV [2], much lower than that of traditional catalysts such as Ni (4.27 eV) and Pt (4.39 eV) [35], effectively avoiding the passivation caused by the deposition of S₈ on the electrode surface. NiS₂ further accelerates the desorption of S₈ through a “sulfur-repellent” surface design (contact angle > 90°), ensuring the continuous progress of the reaction [35].

To address the electrode passivation issue caused by sulfur, a novel SOR pathway has been proposed [29]. The core mechanism of this pathway is as follows: Firstly, HS⁻/S₂⁻ are oxidized to S_n²⁻ on the electrode surface (Equation (1)); subsequently, S_n²⁻ is treated with reagents such as CO₂, HCl, or H₂SO₄ (Equation (2)), ultimately achieving efficient recovery of sulfur. This strategy successfully avoided the key problem of electrode passivation in the direct oxidation recovery process, providing a new direction for the resourceful treatment of sulfur-containing wastewater [29]. In the related research field, Huang et al. were the first to adopt a similar approach [36]. They prepared a ZIS/NiFeS composite through electrochemical deposition, and this electrode could selectively oxidize S²⁻ to S_n²⁻, thereby enabling the collection of S₈. These research results have verified the effectiveness and application potential of this strategy in the treatment of sulfur-containing wastewater from different experimental systems, providing crucial support for its further promotion [3,29]. Additionally, Chen et al.’s research has deeply revealed the multi-path characteristics of the oxidation of HS⁻/S²⁻ to S_n²⁻, and clarified the regulatory effect of HS⁻/S²⁻concentration on the oxidation products [30]. When the S²⁻ concentration is low (100 mg/L), it is more likely to be deeply oxidized to SO₄²⁻, resulting in a significant decrease in the recovery rate of elemental sulfur. When the S²⁻ concentration is increased to 500 mg/L, the oxidation products are mainly S_n²⁻ (such as S₂²⁻ and S₃²⁻), which are more conducive to the subsequent recovery of sulfur. Detailly, under low concentration conditions, HS⁻/S²⁻ is first oxidized to S₂O₃²⁻, and S₂O₃²⁻ is further oxidized to SO₄²⁻; under high concentration conditions, HS⁻/S₂⁻ also initially generates S₂O₃²⁻, but subsequently S₂O₃²⁻ reacts with the excess S²⁻ in the system to form sulfur (S) (Equation (3)), and the generated S then complexes with S²⁻ to form S_n²⁻ (Equation (4)), ultimately creating favorable conditions for the recovery of sulfur.

$${\mathrm{n}\mathrm{S}}^{2-}+{\left(2n-2\right)\mathrm{h}}^{+}\to {\mathrm{S}}_{\mathrm{n}}^{2-}\left(2\le \mathrm{n}\le 4\right)$$

(1)

$${\mathrm{n}\mathrm{S}}^{2-}+{\left(2n-2\right)\mathrm{h}}^{+}\to {\mathrm{S}}_{\mathrm{n}}^{2-}\left(2\le \mathrm{n}\le 4\right)$$

(2)

$${{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+{2\mathrm{S}}^{2-}+{3\mathrm{H}}_2\mathrm{O}\to 4\mathrm{S}+6{\mathrm{O}\mathrm{H}}^{-}$$

(3)

$$\mathrm{S}+{2\mathrm{n}\mathrm{H}\mathrm{S}}^{-}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{S}}_{\mathrm{n}}^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(4)

The reaction efficiency of the direct oxidation pathway mainly depends on the number of active sites on the electrode surface, the adsorption capacity of HS⁻/S²⁻, and the electron transfer efficiency. Generally, electrode materials with high specific surface area and rich defect structures can provide more active sites and promote the adsorption of reactants and electron transfer, thereby enhancing the efficiency of the direct oxidation reaction. In addition, the pH value of the electrolyte significantly affects the direct oxidation pathway [30]. Under alkaline conditions, the dissociation degree of H₂S is high, and the concentration of HS⁻/S²⁻ is large, which is more conducive to the direct oxidation reaction; while under acidic conditions, H₂S mainly exists in molecular form, with weak adsorption capacity, and the direct oxidation reaction rate is lower [37].

3.2. Indirect Oxidation Pathway

The indirect oxidation pathway of SOR, dominated by redox mediators, refers to the oxidation of HS⁻/S²⁻ being accomplished through the oxidation state intermediates generated on the electrode surface as electron transfer mediators [1,38,39]. That is, the electrode first oxidizes the mediator to form an oxidizing intermediate, which then chemically reacts with HS⁻/S²⁻ to oxidize it to elemental sulfur, while the intermediate itself is reduced, forming a cycle of “electrode oxidation of mediator-intermediate oxidation of HS⁻/S²⁻-intermediate regeneration” (Equations (5)–(10)) [3,40,41]. Common indirect oxidation intermediates include high-valent ions such as Fe³⁺, Co³⁺, Ni³⁺, I₃⁻ [29]. Take the indirect oxidation process with Fe³⁺ as the intermediate as an example. Intermediate generation: At the anode, Fe²⁺ is oxidized to Fe³⁺ (Equation (7)). Intermediate oxidation of H₂S: The generated Fe³⁺ enters the electrolyte and undergoes a chemical reaction with H₂S/HS⁻/S²⁻, oxidizing them to sulfur, while Fe³⁺ is reduced to Fe²⁺ (Equation (8)). Intermediate regeneration: The reduced Fe²⁺ re-diffuses to the anode surface and is oxidized back to Fe³⁺, completing the recycling of the intermediate. The overall reaction equation is the same as that of the direct oxidation pathway (Equation (11)).

$${3\mathrm{I}}^{-}+{2\mathrm{h}}^{+}\to {\mathrm{I}}_3^{-}$$

(5)

$${\mathrm{I}}_3^{-}+{\mathrm{S}}^{2-}\to {3\mathrm{I}}^{-}+\mathrm{S}\downarrow$$

(6)

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{h}}^{+}\to {\mathrm{F}\mathrm{e}}^{3+}$$

(7)

$${2\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{S}}^{2-}\to {2\mathrm{F}\mathrm{e}}^{2+}+\mathrm{S}\downarrow$$

(8)

$${\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}-\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{h}}^{+}\to {\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}-\mathrm{F}\mathrm{e}}^{3+}$$

(9)

$${2\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}-\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{S}}^{2-}\to {2\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}-\mathrm{F}\mathrm{e}}^{2+}+\mathrm{S}\downarrow$$

(10)

$$8{\mathrm{S}}^{2-}-{8e}^{-}\to S_8\downarrow$$

(11)

The advantage of the indirect oxidation pathway lies in the existence of intermediates can lower the activation energy of the SOR and increase the reaction rate [29]. Especially for electrode materials with weak electron transfer ability, the indirect oxidation pathway can significantly improve the reaction performance. In addition, the concentration, redox potential, and stability of the intermediates have a significant impact on the efficiency of indirect oxidation [29]. For example, the redox potential of Fe³⁺ (0.77 V vs. SHE) is moderate, which can effectively oxidize sulfide without easily causing over-oxidation to form SO₄²⁻; while the redox potential of Co³⁺ is relatively high (1.81 V vs. SHE), although it has strong oxidation ability, it is prone to over-oxidation of sulfide, reducing the selectivity of sulfur [42].

4. Analysis of Sulfur Oxidation Products

The products of SOR are complex and diverse, encompassing not only the target product S₈ but also a range of intermediates and by-products such as S_n²⁻, S₂O₃²⁻, SO₃²⁻, and SO₄²⁻. Accurate analysis and identification of the types, contents, and existing forms of these sulfur oxidation products hold crucial significance for optimizing reaction conditions, improving SOR selectivity, and elucidating the underlying reaction mechanism. At present, the main analytical methods for SOR products are XRD, Raman, UV-vis spectrometer (UV-vis), and gas chromatography-mass spectrometry (Figure 2) [3,43].

The SOR value-added product S₈ is usually confirmed by XRD and Raman analysis. For example, Xiao et al. analyzed the SOR product by XRD and found that the collected product powder corresponded to sulfur (S₈, PDF #78-1888) [2]. Chen et al. analyzed the light yellow powder collected from SOR by XRD and found that the product powder corresponded to elemental sulfur (S₈, PDF #74-1465) [30]. In addition, Raman spectroscopy is also an effective method for analyzing sulfur. Xiao et al. found that when NiMoN@C/NF was used as the SOR anode, a new characteristic peak of S₈ appeared at 461 cm⁻¹ when the SOR voltage rose to 0.4 V (vs. RHE), and three characteristic peaks of S₈ at 146, 216, and 461 cm⁻¹ were observed when the applied voltage was greater than 0.45 V (vs. RHE) [2]. Similarly, He et al. constructed an in situ electrochemical Raman system using a self-made in situ Raman device [44]. During the potentiation test at a constant current of 50 mA/cm², a laser wavelength of 532 nm was used for testing every minute, and the absorption peak of S₈ was detected at a Raman wavelength of 220 cm⁻¹.

To gain a deeper understanding of the formation pathway of S₈, it is necessary to conduct an in-depth analysis of the intermediate products of SOR. In the direct oxidation pathway (* + S²⁻→*S²⁻→*S₂²⁻→*S₃²⁻→*S₄²⁻→*S₈²⁻→* + S₈), the main intermediate products are S_n²⁻. The adsorbed intermediate products * S_n²⁻ are mainly detected by in situ Raman spectroscopy. He et al. used in situ Raman spectroscopy with an excitation wavelength of 532 nm and detected S_n²⁻ at 450~550 nm, directly proving the pathway of SOR to produce S₈ [44]. However, for free S_n²⁻, UV-vis spectrophotometry is currently used for detection. Chen et al. used a UV-vis (UV-2100, Shanghai Mapada Instruments Co., Ltd., Shanghai, China) and detected S_n²⁻ (2≤ n ≤4) at absorption wavelengths of 300 and 370 nm [2]. However, UV-vis cannot determine the specific value of n in S_n²⁻. Zheng et al.’s research shows that GC-MS can be used to accurately analyze S_n²⁻. This study used trifluoromethanesulfonic acid methyl ester (CF₃SO₃CH₃) as a methylation reagent to react with S_n²⁻ (Equation (12)) [45]. CF₃SO₃CH₃ can convert S_n²⁻ into (CH₃)₂S_n. The reaction principle is that the methyl group (-CH₃) in CF₃SO₃CH₃ undergoes methylation with S_n²⁻, and then the generated (CH₃)₂S_n is determined by GC-MS [43]. For example, Chen et al. used this method to precisely distinguish between two S²⁻ oxidation products, namely (CH₃)₂S₂ (m/z = 93.9910) and (CH₃)₂S₃ (m/z = 125.9631) [30]. In addition to the above core species, the analysis methods for other sulfur species in the SOR system have been well established: S²⁻ was tested by the spectrophotometric method using p-amino dimethylaniline. S₂O₃²⁻ is commonly determined by titration techniques such as iodometry, while the concentrations of SO₃²⁻ and SO₄²⁻ in solution were analyzed using ion chromatography [30,46].

$${\mathrm{S}}_{\mathrm{n}}^{2-}+2{\mathrm{C}\mathrm{F}}_3{\mathrm{S}\mathrm{O}}_3{\mathrm{C}\mathrm{H}}_3\to \left({\mathrm{C}\mathrm{H}}_3\right){\mathrm{S}}_{\mathrm{n}}+{2\mathrm{C}\mathrm{F}}_3{\mathrm{S}\mathrm{O}}_3^{-}$$

(12)

5. Electrodes for Recovering Sulfur

In the SOR process, the electrodes serve as the core carriers for electron transfer. Their material composition, microstructure, and surface properties directly determine the oxidation pathway of S²⁻, reaction kinetics, and product selectivity [3]. Based on differences in the target products of SOR and reaction mechanisms, electrodes for sulfur recovery can be categorized into three main types (

Table 1. Different types of electrodes for SOR.

Electrodes Type	Anodes	Active Species of SOR	Products of SOR	References
Direct oxidation electrodes	CoNi@NGs	Co3+/Co2+, Ni3+/Ni2+	S8	[47]
	NiFeS/ZnIn2S4	Ni3+/Ni2+	S8	[48]
	Pd8Co2/C	Pd2+/Pd0, Co3+/Co2+, OHads	S8	[49]
	Ti/IrO2-Ta2O5	Ir4+/Ir3+	S8	[50]
	MMO/Ti	O2, OH	S0, S2O32−, SO42−, SO32−	[51]
	Ir/Ta/Ti	Ir4+/Ir3+, OH/O2	S0, SO42−, S2O32−	[11]
Controllable Sn2− producing electrodes	CuCoNiMnCrSx/NF	Cu2+/Cu+, Mn3+/Cr3+	Sn2−	[33]
	Cu2S/NF	Cu2+/Cu+	Sn2−	[52]
	Sn4+-doped α-Fe2O3	Fe3+/Fe2+	Sn2−	[53]
	Bi2S3	Bi3+/Bi2+	Sn2−	[54]
	HEA-Mo2C/HPC	Sx2−/Sx+12−	Sn2−	[55]
	NF/Mo-Co(OH)2	Co(OH)2	Sn2−	[56]
Medium circulation-driven indirect oxidation electrodes	Graphite felt	I−/I3−	S8	[57]
	WO3/SiPVC	I−/I3−	S8	[1]
	WO3/FTO	I−/I3−	S0	[40]
	n-Si@PEDOT/p-Si@Pt	Fe3+/Fe2+	S0	[41]
	G/GCS	EDTA-Fe3+/Fe2+	S0	[58]

): (1) high-efficiency sulfur-producing electrodes, (2) controllable S²⁻-producing electrodes, and (3) medium circulation-driven indirect oxidation electrodes.

5.1. Direct Oxidation Electrodes

Direct oxidation-type electrodes can efficiently catalyze S²⁻ to directly generate S₈ through one or multiple electron transfer steps [3,29]. Due to the advantages of short reaction paths, low energy loss, and high product selectivity, they have become the primary focus of current research on electrodes for elemental sulfur recovery [33,59]. The key to their design lies in constructing electrode surfaces with high active site density, strong S²⁻ adsorption capacity, and excellent electronic conductivity, while suppressing the formation of by-products such as S₂O₃²⁻ and SO₄²⁻ [3,60,61,62]. Currently, the more mature types mainly include transition metal sulfide (TMS) electrodes, alloy and high-entropy alloy (HEA) electrodes, metal-carbon composite electrodes, and noble metal sulfide electrodes [3,33]. Among them, TMS electrodes follow the “adsorption-electron transfer-polymerization-desorption” path (S²⁻→*S→*S₂→…→*S₈→S₈), and by regulating the strength of the metal-sulfur (M-S) bond to inhibit passivation [63,64]. For example, NiS₂ has a “sulfur-repellent” surface that can repel S₈ deposition, has an overpotential of only 0.41 V at 10 mA/cm² in 0.05 mol/L Na₂S electrolyte, a sulfur recovery rate of 92%, and no obvious passivation after 100 h of operation [35]. Ni₃S₂, after Co doping to regulate the d-band center, can shorten the oxidation path, and the cell voltage drops to 0.80 V at 50 mA/cm² [65]. Alloy and high-entropy alloy electrodes regulate the d-band center through alloying to balance S²⁻ adsorption and S₈ desorption [66]. High-entropy alloys can also enhance structural stability by high configurational entropy [67,68,69,70,71]. For example, Pd₈Co₂/C has an overpotential of −0.35 V (vs. Ag/AgCl) in 1 mol/L Na₂S + 3 mol/L KOH at 100 mA/cm², enabling the maximum power density of direct alkaline sulfur fuel cells to reach 46.82 mW/cm² (26% higher than Pd/C) [49], and Pd₉Ir₁/C has a power density of 33.98 mW/cm² at 70 °C [72]. The FeCoNiCuMn HEA and Mo₂C composite electrode, due to the synergy of multiple elements and the low SOR energy barrier of the Mo₂C (102) crystal plane, has an overpotential of 0.382 V at 100 mA/cm² and is suitable for both HER and SOR [55]. Metal-carbon composite electrodes rely on carbon substrates to promote electron conduction and mass diffusion and utilize the charge transfer effect at the metal-carbon interface to optimize the S²⁻ adsorption energy barrier [3,73,74]. For example, CoNi@NGs has an initial SOR potential of 0.25 V and a current density twice that of Pt/C in 1 mol/L Na₂S + 1 mol/L KOH [47].

5.2. Controllable S_n²⁻ Producing Electrodes

Controllable S_n²⁻-producing electrodes aim to selectively catalyze the oxidation of S²⁻/HS⁻ to specific S_n²⁻ as the core objective [3,30]. By precisely regulating the reaction pathway to avoid over-oxidation to by-products such as SO₃²⁻ and SO₄²⁻, they provide controllable intermediates for subsequent stepwise conversion to elemental sulfur [3,35,36,44]. The key to their design lies in matching the energy barrier requirements for the stepwise oxidation of S²⁻ and enhancing product selectivity [3]. Typical electrode materials include transition metal sulfides, doped metal oxides, and high-entropy sulfides [3]. For example, Cu₂S/NF electrodes utilize the Cu⁺/Cu²⁺ redox couple to modulate electron transfer, limiting S²⁻ oxidation to the S_n²⁻ stage by controlling the electron transfer rate [52]; Sn⁴⁺-doped α-Fe₂O₃ introduces defect sites via doping while leveraging the Fe³⁺/Fe²⁺ redox activity, enhancing S²⁻ adsorption and inhibiting deep oxidation [53]; high-entropy sulfide CuCoNiMnCrS_x/NF achieves precise regulation of S_n²⁻ chain length (mainly n = 2~4) through the synergistic effect of multiple elements, which optimizes the d-band center to balance S²⁻ adsorption and S_n²⁻ desorption [33]; B₂S₃ electrodes rely on the valence change of Bi³⁺/Bi²⁺ to adjust catalytic activity, and preferentially generate short-chain Sn²⁻ under mild potentials (0.2~0.4 V vs. RHE) [54]. Additionally, electrodes like NF/Mo-Co(OH)₂ use Co(OH)₂’s surface hydroxyl groups to selectively adsorb S²⁻, and the intercalated structure restricts the growth of polysulfide chains, further ensuring the selectivity of S_n²⁻ [56]. Such electrodes are particularly suitable for sulfur-containing wastewater treatment systems, as the generated S_n²⁻ can be efficiently converted to S₈ by adding acids (e.g., HCl, H₂SO₄) in subsequent processes, avoiding electrode passivation caused by direct S₈ deposition, and their performance can be further optimized by adjusting electrolyte pH (alkaline conditions are more favorable for maintaining S_n²⁻ stability and preventing disproportionation) [30].

5.3. Medium Circulation-Driven Indirect Oxidation Electrodes

Indirect oxidation electrodes do not directly react with S²⁻ but instead achieve the conversion of S²⁻ to S₈ through catalyzing the redox cycle of redox mediators [40,41,58]. This system has the advantages of easy product separation and less electrode passivation, making it suitable for treating sulfide wastewater with high concentration and complex composition [3]. According to the type of redox mediators, it can be classified into chemical mediators, indirect oxidation electrodes, and biological mediators indirect oxidation electrodes [29,75,76].

Indirect oxidation systems with chemical redox mediators typically employ transition metal ions (such as Fe³⁺, EDTA-Fe³⁺, I₃⁻, etc.) or organic compounds (such as quinones) as redox mediators [3]. The role of the electrode is to oxidize the reduced state mediator (such as Fe²⁺, EDTA-Fe²⁺, I⁻, etc.) to the oxidized state, and the oxidized state mediator then reacts with S²⁻ in the solution to form S₈. For example, in the Fe³⁺/Fe²⁺ mediator system, the titanium-based lead dioxide (Ti/PbO₂) electrode, due to its high oxygen overpotential, can efficiently catalyze the oxidation of Fe²⁺ to Fe³⁺. At a current density of 20 mA/cm², the oxidation rate of Fe²⁺ reaches 99%, and the generated Fe³⁺ reacts with S²⁻ to form S₈, with a total sulfur recovery rate exceeding 90% [51,77,78]. In addition, organic quinone mediators (such as anthraquinone-2-sulfonic acid) have the characteristic of adjustable redox potential. After modification of the glassy carbon electrode, it can catalyze the oxidation of quinones, indirectly achieving the mild oxidation of S²⁻, with a selectivity for S₈ reaching 92% [1,76,79].

Biological mediator indirect oxidation electrodes use microorganisms as redox mediators [75,80]. Through the metabolic activities of microorganisms, S²⁻ is oxidized to S₈, and the electrode acts as an electron acceptor for the microorganisms, facilitating the electron transfer in the metabolic process. Such electrodes are typically biofilm electrodes, composed of a conductive substrate (such as carbon felt or graphite rods) and a microbial film. For instance, a carbon felt bioelectrode with Thiobacillus thiooxidans attached, under the catalytic action of microorganisms, S²⁻ is first oxidized to S_n²⁻ and then further converted to S₈ [75]. At a low voltage (0.2 V vs. RHE), the sulfur recovery rate reaches 88%, and the energy consumption is only 1/3 of that of chemical oxidation methods [75]. In addition, microbial fuel cell-type indirect oxidation electrodes can achieve the synergy of energy production and sulfur recovery [81]. While the electrode catalyzes the microbial metabolism to generate electricity, it indirectly promotes the oxidation of S²⁻ to S₈. For instance, a carbon paper electrode with Shewanella oneidensis as the microorganism can achieve an S₈ production at a power density of 0.66 mW/cm² [82]. The key challenge in the bio-mediated system lies in the stability of the microorganisms. By using immobilization techniques (such as sodium alginate entrapment), the adhesion strength of the microorganisms on the electrode surface can be enhanced, thereby extending the service life of the electrode [83].

6. Electrochemical Coupling System for SOR

SOR is not only the core process for recovering elemental sulfur, but also its accompanying electron transfer characteristics and oxidation capacity endow it with great potential for coupling with fields such as energy production and pollution control [29]. By constructing a “sulfur oxidation-targeted function” collaborative system, multiple goals such as “resource recovery-energy output-pollution removal” can be achieved, significantly enhancing the comprehensive value of the sulfur oxidation process [2]. Currently, the mainstream research systems mainly fall into two categories (

Table 2. Different electrochemical systems for SOR.

Electrochemical Coupling Systems	Anodes	Products of SOR	Cathodes	Products in Cathodes	References
SOR coupled with the production of energy substances	Sn4+-doped α-Fe2O3	Sn2−	Pt	H2	[53]
	Bi2S3	Sn2−	Pt	H2	[54]
	WO3/SiPVC	Sn2−	GDE	H2O2	[1]
	p-Si	S	TiO2/Ti/n+p-Si	H2O2	[38]
	WO3/FTO	S0	Pt/SiPVC	H2	[40]
	Cu2S/NF	Sn2−	Pt/Gr	H2	[52]
	HEA-Mo2C/HPC	Sn2−	HEA-Mo2C/HPC	H2	[55]
	CoS-NF	Sn2−	CoGa-NS-C	H2O2	[84]
	CoNiS2/NF	S0/S2O32−	Pt	H2	[85]
	a/c S-Pd NSA/NF	Sn2−	a/c S-Pd NSA/NF	H2	[86]
	NiS2	S0	NiS2	H2	[35]
	Co-S NSs	Sn2−	p-Bi NSs	HCOOH	[13]
	n-Si@PEDOT/p-Si@Pt	S0	p-Si@Pt	H2	[41]
	NiS–CoS/CNF	S0	Pt	H2	[31]
	G/GCS	S0	ZnO@Gr	CO	[58]
SOR coupled with oxidizing pollutants treatment	Ru-Ni(OH)2/NF	S0	Ru-Ni(OH)2/NF	NH3	[87]
	Pd-Co@NC/CC	S0	Pd–Co@NC/CC	NH3	[88]
	MOF-derived CoSx	S0	MOF-derived CoSx	NH3	[89]
	Ni-MoS2@ACF	S0	Ni-MoS2@ACF	NH3	[90]
	CoNiOOH/CC	S0	CoNiOOH/CC	NH3	[91]
	AuCu/CuS NWs	S0	AuCu/CuS NWs	NH3	[92]
	Cu-NiO UTNSs	S0	Cu–NiO UTNSs	NH3	[93]
	Gr	S0	Gr	VO2+	[94]
	Gr	S0	Gr	Cr3+	[95]

): sulfur oxidation coupled with energy substance production and sulfur oxidation coupled with pollutant treatment.

6.1. SOR Coupled with the Production of Energy Substances

Electrocatalytic sulfur oxidation coupled with energy production takes the electrocatalytic system as the core. It releases electrons through the electrocatalytic oxidation of S²⁻ at the anode and combines with the reduction reaction at the cathode (such as oxygen reduction, hydrogen evolution, and CO₂ reduction) to achieve the targeted production of clean energy (H₂, H₂O₂, CH₄, CO, CHOOH, etc.) [29,30,33]. The key to this system lies in optimizing the activity and selectivity of the anode electrocatalyst, reducing the reaction overpotential, and matching efficient cathode reactions to enhance energy conversion efficiency [29]. Currently, mature coupling modes include electrocatalytic SOR coupled with ORR, electrocatalytic SOR coupled with HER, and electrocatalytic SOR coupled with CO₂RR, as shown in Figure 3.

6.1.1. SOR Coupled with ORR

There exists a thermodynamic driving force for the spontaneous reaction between S²⁻ and O₂, which provides a thermodynamic foundation for the efficient coupling of the electrocatalytic SOR and ORR [96]. From the perspective of thermodynamic matching, the potential intervals and reaction energy barriers of the two reactions are highly consistent [30]. From a kinetic perspective, by rationally designing the catalyst and reaction system, the electron transfer rate of SOR and the reaction path of ORR can be effectively regulated to achieve a coordinated alignment of the reaction rates at both electrodes, thereby ensuring the stable operation of the entire coupled system [30,38,40]. At the cathode, ORR mainly occurs through two reaction pathways: 4e⁻-ORR and 2e⁻-ORR [30,97]. The standard reduction potential of the 2e⁻-ORR pathway is 0.68 V (vs. RHE), which is much lower than that of the 4e⁻-ORR pathway at 1.23 V (vs. RHE) [98,99,100]. This indicates that the 2e⁻-ORR pathway has a lower reaction energy barrier thermodynamically and is more likely to occur preferentially [30]. Meanwhile, the product of 2e⁻-ORR is H₂O_2, an environmentally friendly oxidant with strong oxidation capacity and without secondary pollution after reaction [101,102]. It has shown broad application in various fields such as medical disinfection, advanced oxidation treatment of environmental pollutants, energy storage (such as H₂O₂ fuel cells), and fine chemical synthesis [103,104]. This makes the SOR-2e⁻-ORR coupled system have dual significance in environmental governance and the preparation of high-value chemicals [38]. Typically, Zong et al. constructed a SOR-2e⁻-ORR system (Figure 3a), combining the AQ/H₂AQ and I⁻/I₃⁻ redox couples, which is based on an unbiased solar photoelectrochemical cell. The cell uses a Nafion membrane to separate the anodic and cathodic compartments, confining the two redox couples, respectively, to avoid backward reactions. This system can utilize solar energy to oxidize toxic H₂S to elemental sulfur through the I⁻/I₃⁻ redox couple and O₂ to valuable H₂O₂ via the AQ/H₂AQ redox couple, with both processes achieving high Faradaic efficiencies [38].

6.1.2. SOR Coupled with HER

The electrocatalytic SOR-assisted H₂ production system replaces the traditional oxygen evolution reaction (OER) in water electrolysis with the electrocatalytic oxidation of S²⁻ at the anode, taking advantage of the fact that the oxidation potential of S²⁻ (−0.48 V vs. RHE) is much lower than that of OER (1.23 V vs. RHE), significantly reducing the cell voltage and energy consumption [2,3,44]. The core of this system is to develop anode electrocatalysts with both high activity and stability while matching them with efficient hydrogen evolution cathodes to achieve simultaneous optimization of S²⁻ oxidation and H₂ production. In an acidic system, an electrolytic system with defective CoCd_(x:y)S_n as the anode and Pt as the cathode has a cell voltage of only 0.25 V, reducing energy consumption, with a high efficiency S²⁻ recovery and a stable H₂ production with an achieved 98% H₂ Faradaic efficiency with remarkable stability of 120 h [105]. Under alkaline conditions, an electrolytic system with lamellar stacking carbon-confined nickel hybrid nanosheets (TLNi@C HNSs) as bifunctional electrodes (both anode and cathode) has a cell voltage of only 0.91 V at an industrial current density of 1 A cm⁻², reducing energy consumption (electricity expense of 2.1 kWh h⁻¹ m⁻²), with efficient S²⁻ conversion to sulfur (achieving 3.83 kg h⁻¹ m⁻² S₈ productivity and 69% S₈ yield) and stable H₂ production, with remarkable stability of over 300 h [106]. Additionally, by constructing a flow-type electrolytic cell system, mass transfer of the electrolyte can be enhanced, reducing the deposition and passivation of S₈ on the electrode surface. Zhu et al. constructed a flow-type electrolytic cell system based on anion exchange membranes [106]. By continuously supplying an alkaline electrolyte containing S²⁻ (1.0 M KOH + Na₂S) and promptly removing the products on the electrode surface, the mass transfer efficiency of the electrolyte was effectively enhanced, and the deposition and passivation of S₈ on the electrode surface were suppressed. This system not only achieved a high current density of 200 mA cm⁻² but also reached an industrial-level current density of 1 A cm⁻², while significantly reducing the energy consumption per unit hydrogen production.

6.1.3. SOR Coupled with CO₂RR

The electrocatalytic SOR-CO₂RR coupling system utilizes the electrons generated from the oxidation of S²⁻ at the anode to drive the reduction of CO₂ at the cathode to produce carbon-based fuels such as CO, CH₄, or CHOOH, achieving the triple goals of “sulfur recovery-carbon emission reduction-energy production” [13,58]. Core to this system is the design of high-performance, low-cost catalysts for both half-reactions: for cathodic CO₂RR, non-precious metal catalysts such as graphene-encapsulated ZnO (ZnO@G) and highly dispersed p-Bi nanosheets (p-Bi NSs) have shown excellent selectivity—ZnO@G achieves a Faradaic efficiency for CO of up to 83% and a turnover frequency of 0.024 s⁻¹ at −0.813 V vs. RHE, while p-Bi NSs maintains over 90% FE for formate across a wide potential range (−0.6~1.1 V vs. RHE) [13,58]; for anodic SOR, graphene-modified graphite carbon sheets (G/GCS) and porous Co-S nanosheets (Co-S NSs) exhibit high activity, with G/GCS enabling 100% FE for EDTA-Fe²⁺ oxidation and Co-S NSs achieving an ultra-low SOR onset potential of 0.2 V vs. RHE [13,58]. The technical challenge of this system lies in matching the electron transfer rates of the anodic S²⁻ oxidation and cathodic CO₂ reduction while suppressing side reactions such as HER at the cathode [57,58]. To address this, researchers have optimized reactor configurations and redox mediators: for example, an “H-type” electrolyzer using EDTA-Fe²⁺/Fe³⁺ as a redox couple (with 182-times lower membrane crossover than bare Fe²⁺/Fe³⁺) achieves stable co-production of CO (0.13 mmol·h⁻¹·cm⁻²) and S at a current density of 8.5 mA·cm⁻² [58]; a dual-flow electrolyzer with ultrathin interelectrode distance and CoPc-loaded gas diffusion electrodes (GDE) further boosts CO production rate by 7-fold and energy efficiency to 72.41%, while reducing the cost of natural gas purification by 0.03 $ per cubic meter [29,57]. Additionally, integrating solar energy via three-junction Si solar cells enables a solar-to-chemical (STC) conversion efficiency of 7.5–8.3%, and a novel CO₂-assisted sulfur separation strategy avoids corrosive acids. CO2 adjusts electrolyte acidity to precipitate S₈, with by-product NaHCO₃, reused as CO₂RR electrolyte, significantly enhancing atom utilization [13,58]. Practically, the system has been validated with simulated natural gas (Ar:CH₄:CO₂:H₂S = 60:20:19:1 v/v), successfully converting concomitant CO₂ and H₂S into CO and S [58]. A two-electrode flow cell configuration even achieves 100 mA·cm⁻² at only 1.5 V, far lower than conventional CO₂RR-OER systems (~1.83 V), demonstrating great potential for industrial-scale carbon-neutral and resource-recycling applications [13,58].

6.2. SOR Coupled with Oxidizing Pollutants Treatment

Electrocatalytic SOR coupled with pollutant treatment (Figure 4), anodic oxidation for degrading sulfur-containing wastewater, and cathodic reduction in heavy metals and NO₃⁻ [87,94]. This system can precisely control the oxidation capacity and reaction pathways by adjusting parameters such as applied voltage and current density and is suitable for the simultaneous treatment of complex multi-component wastewater [94].

The coupling of reductive wastewater treatment in an electrochemical system refers to the oxidation and degradation of S²⁻ at the anode and the reduction in heavy metals, NO₃⁻, or other oxidizing pollutants at the cathode during the operation of the electrochemical system. Specifically, the anode converts S²⁻/HS⁻ into S₈ through direct or indirect oxidation pathways, while the cathode utilizes electrons generated from anodic SOR to drive the reduction in pollutants (such as nitrate/nitrite (NO₃⁻/NO₂⁻) to ammonia (NH₃), hexavalent chromium (Cr⁶⁺) to low-toxic trivalent chromium (Cr³⁺), and vanadium ions (VO₂⁺) to VO²⁺, realizing both pollutant detoxification and resource recovery [93,94,95]. This system is similar to the coupling of reductive wastewater treatment with the production of energy substances, but the difference is that heavy metals, NO₃⁻, or other oxidizing pollutants are used as electron acceptors at the cathode instead of reactions like HER or CO₂RR [29]. Typically, anion or cation exchange membranes are employed to separate the anode and cathode compartments to prevent cross-reaction between oxidizing and reducing species while maintaining charge balance. Through this system, both reductive sulfur-containing wastewater and oxidizing heavy metal/nitrate-containing wastewater can be treated simultaneously, making it suitable for remediating complex multi-component industrial wastewater.

7. Conclusions and Perspectives

7.1. Conclusions

Hydrogen sulfide (H₂S), a toxic by-product of fossil fuel processing and biomass energy development, poses severe threats to ecological stability, human health, and industrial infrastructure-yet its sulfur component represents a valuable resource with global annual demand exceeding 70 million tons. This review systematically synthesizes the latest progress in electrocatalytic H₂S splitting for sulfur recovery, focusing on the sulfide oxidation reaction (SOR) as the core process, and reveals how electrochemical coupling systems have emerged as a transformative solution to address both H₂S pollution and resource-energy nexus challenges.

From a mechanistic perspective, SOR pathways are clarified as two complementary routes: direct oxidation, where S²⁻/HS⁻ undergoes stepwise polymerization on anode surfaces to form S₈ (with S* desorption as the rate-limiting step), and indirect oxidation, which leverages redox mediators (e.g., Fe³⁺/Fe²⁺, I⁻/I₃⁻) to mitigate the primary bottleneck of electrode passivation caused by high-resistance S₈ deposition. Accurate identification of SOR products-ranging from target S₈ to intermediates (S_n²⁻) and by-products (S₂O₃²⁻, SO₄²⁻)-has been enabled by advanced analytical techniques, including XRD, Raman spectroscopy, UV-vis spectrophotometry, and GC-MS; these methods not only validate product selectivity but also provide critical insights into reaction pathway dynamics, laying the groundwork for optimizing SOR conditions.

In terms of material design, SOR catalysts have evolved beyond traditional noble metals to include earth-abundant alternatives, such as transition metal sulfides (TMSs, e.g., NiS₂ with “sulfur-repellent” surfaces), alloys (Pd-Co, high-entropy alloys), and metal-carbon composites (CoNi@NGs). The key design principle across these materials is modulating sulfur affinity: alloying and defect engineering tailor the d-band center to balance S²⁻ adsorption and S₈ desorption, while structural innovations (e.g., porous nanoarrays, heterojunctions) enhance electron transfer and mass diffusion. These advances have enabled catalysts to achieve high sulfur recovery rates (up to 92%) and long-term stability (over 100 h of operation), addressing critical performance limitations of early-stage materials.

Most notably, electrochemical coupling systems centered on SOR have expanded the technology’s value beyond sulfur recovery alone. By integrating SOR with cathode reactions such as hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), and oxygen reduction reaction (ORR), these systems simultaneously achieve “waste detoxification-sulfur recovery-high-value production”: for example, SOR-assisted HER reduces cell voltage to as low as 0.25 V (vs. traditional water electrolysis), while SOR-CO₂RR co-produces CO (Faradaic efficiency > 83%) and S₈, aligning with global “carbon peak and carbon neutrality” goals. For pollutant treatment, coupled systems also enable simultaneous remediation of sulfur-containing wastewater and reduction in oxidizing pollutants (e.g., NO₃⁻ to NH₃, Cr⁶⁺ to Cr³⁺), demonstrating broad applicability in complex industrial scenarios.

Collectively, these advances highlight that electrocatalytic H₂S splitting is no longer merely a pollution control technology but a multi-functional platform that bridges environmental protection, resource recycling, and energy production. Its ability to operate under mild conditions (ambient temperature/pressure) and adapt to low-concentration H₂S (<5 vol%) addresses inherent limitations of traditional Claus processes, making it a promising candidate for industrial scaling.

7.2. Perspectives

Despite significant progress, electrocatalytic H₂S splitting for sulfur recovery remains constrained by gaps between fundamental research and industrial application, and future efforts must focus on resolving core challenges while expanding the technology’s impact.

First, mechanistic understanding of SOR and passivation requires atomic-level resolution. Current studies have identified S_n²⁻ as a key intermediate, but dynamic changes in adsorbed species (e.g., S₃* and S₃*) during reaction, as well as their interactions with catalyst surfaces, remain poorly characterized. Integrating in situ characterization techniques (e.g., in situ Raman/ATR-FTIR) with density functional theory (DFT) calculations will enable real-time tracking of intermediate evolution and quantification of energy barriers for rate-limiting steps. Furthermore, machine learning approaches can correlate catalyst electronic properties (d-band center, adsorption energy) with sulfur tolerance, establishing universal descriptors to accelerate the screening of anti-passivation catalysts—an advancement that would reduce the trial-and-error nature of current material design.

Second, catalyst innovation must prioritize low cost, sulfur resistance, and multifunctionality. While TMSs and high-entropy alloys show promise, their scalability is limited by synthesis complexity and reliance on rare elements (e.g., Pd in Pd-Co alloys). Future research should focus on earth-abundant elements (Fe, Co, Ni) and develop scalable synthesis methods (e.g., electrochemical deposition, hydrothermal synthesis) for high-performance catalysts. Structural design should further integrate “sulfur-repellent” surfaces (e.g., NiS₂’s contact angle > 90°) with defect engineering to enhance both activity and stability; for example, doping Sn⁴⁺ into α-Fe₂O₃ introduces defect sites that inhibit deep oxidation of S²⁻ to SO₄²⁻, a strategy that could be extended to other metal oxides. Additionally, developing bifunctional catalysts (e.g., TLNi@C HNSs for both SOR and HER) will simplify system design and reduce costs, a critical requirement for industrial adoption.

Third, electrochemical coupling systems need to advance toward integration and industrial adaptation. Current lab-scale systems often use batch reactors and ideal feedstocks (e.g., pure Na₂S solutions), but industrial streams contain impurities (e.g., heavy metals in refinery wastewater) that degrade catalyst performance. Future systems should adopt membrane electrode assemblies (MEAs) and flow reactors to enhance mass transfer, suppress S₈ deposition, and handle complex feedstocks; for instance, flow-type electrolyzers with anion exchange membranes have achieved industrial-level current densities (1 A cm⁻²) while maintaining high sulfur recovery. Redox mediator circulation (e.g., EDTA-Fe³⁺/Fe²⁺ with reduced membrane crossover) and CO₂-assisted sulfur separation (using CO₂ to adjust pH for S₈ precipitation, with NaHCO₃ reused as CO₂RR electrolyte) can further improve atom efficiency and reduce secondary waste. Integrating solar energy (e.g., three-junction Si solar cells for photoelectrochemical systems) will also lower energy consumption, aligning the technology with renewable energy transitions.

Finally, the broader impact of this field lies in its potential to address interconnected global challenges: H₂S pollution control, sulfur resource scarcity, and clean energy demand. By coupling SOR with waste-to-energy processes (e.g., microbial fuel cells for simultaneous electricity generation and sulfur recovery), the technology can contribute to circular economy models in the oil, gas, and coal chemical industries. Long-term vision should include cross-disciplinary collaboration-combining electrochemistry, materials science, and environmental engineering-to develop modular, scalable systems that can be deployed in both centralized (e.g., refineries) and decentralized (e.g., small-scale biogas plants) settings.

In summary, electrocatalytic H₂S splitting for sulfur recovery stands at a critical juncture: with continued advances in mechanistic understanding, catalyst design, and system integration, it has the potential to become a cornerstone technology for sustainable pollution management and resource-energy synergy.