Recent Advances in the Electron Transfer Mechanism of Fe-Based Electro-Fenton Catalysts for Emerging Organic Contaminant Degradation

Abstract

Heterogeneous electro-Fenton (HEF) technology utilizing iron-based cathode catalysts has emerged as an efficient advanced oxidation process for wastewater treatment, demonstrating outstanding performance in degrading emerging organic contaminants (EOCs) while maintaining environmental sustainability. The performance of this technology is governed by two critical processes: the accumulation of H₂O₂ and the electron transfer mechanisms governing the Fe(III)/Fe(II) redox cycle. This review comprehensively summarizes recent advances in understanding the electron transfer mechanisms in iron-based HEF systems and their applications for EOC degradation. Five representative catalyst categories are critically analyzed, including zero-valent iron/alloys, iron oxides, iron-carbon/nitrogen-doped carbon composites, iron sulfides/phosphides, and iron-based MOFs, with a particular focus on their structural design, catalytic performance, and electron transfer mechanisms. A particular focus is placed on strategies enhancing Fe(III)/Fe(II) cycling efficiency and the interplay between radical (^•OH) and non-radical (¹O₂) oxidation pathways, including their synergistic effects in complex wastewater systems. Major challenges, including catalyst stability, pH adaptability, and selective oxidation in complex matrices, are further discussed. Potential solutions to these limitations are also discussed. This review provides fundamental insights for designing high-efficiency iron-based HEF catalysts and outlines future research directions to advance practical applications.

1. Introduction

Emerging organic contaminants (EOCs), including antibiotics, pesticides, dyes, polycyclic aromatic hydrocarbons (PAHs), perfluorooctanoic acid (PFOA), and polychlorinated biphenyls (PCBs), have posed severe threats to aquatic ecosystems and human health due to their environmental persistence, bio-toxicity, and recalcitrance to degradation [1,2,3]. Conventional wastewater treatment plants (WWTPs) exhibit limited removal efficiency for EOCs, primarily attributed to their high hydrophilicity, low absorbability, and resistance to biodegradation [4,5]. Besides increasing contamination to the environment, the adverse effects of biological magnification and bioaccumulation cannot be ignored [6]. Consequently, the development of cost-effective, efficient, and environmentally friendly innovative water treatment technologies for EOC removal is imperative to address this growing environmental challenge.

Among the available strategies, advanced oxidation processes (AOPs) have garnered significant attention for addressing EOCs owing to their non-selectivity, strong oxidative capacity, and capability for complete mineralization [5,7]. In particular, the electro-Fenton (EF) process, a prominent electrochemical AOP, has demonstrated remarkable efficacy in generating highly reactive hydroxyl radicals (^•OH) to degrade organic pollutants, benefiting from its high versatility, operational flexibility, ease of automation, cost-effectiveness, and environmental compatibility [8,9,10]. Conventionally, the EF process can be categorized into homogeneous and heterogeneous variants. However, the homogeneous EF system is often constrained by the stringent pH dependence (pH 2–4) and the generation of undesirable metal sludge [11]. To address these inherent limitations of EF, recent research has shifted toward heterogeneous systems [12]. The heterogeneous electro-Fenton (HEF) technology has been developed as a promising alternative; it expands the applicable pH range of the reaction and effectively eliminates the generation of metal sludge [13,14]. To date, numerous HEF catalysts have been investigated, including iron-based, cobalt-based, copper-based, clay-based, and molybdenum-based catalysts, along with carbonaceous materials and metal-organic framework-derived composites, etc. [15,16,17,18,19,20]. Among these, iron-based catalysts have emerged as particularly promising due to their environmental compatibility, non-toxicity, low cost, and wide availability [21]. Consequently, iron-based catalysts have emerged as one of the most active and promising frontiers in the realm of HEF research.

Although HEF systems theoretically overcome the drawbacks of EF, their practical efficiency is governed by complex interfacial processes [12,22]. The efficiency of HEF systems largely depends on the concurrent accumulation of H₂O₂ and the reduction of Fe(III) [23,24]. A critical issue arises from the competitive interplay between the oxygen reduction reaction (ORR) and Fe(III) reduction at the cathode, which severely compromises the overall treatment efficiency [25,26]. On one hand, excessive H₂O₂ production implies ORR dominance at the cathode, which suppresses Fe(III) reduction and undermines the system, as well as reduces its long-term performance [27]. On the other hand, while elevated H₂O₂ levels promote ^•OH generation, the short half-life of ^•OH (t_1/2~20 ns) leads to the self-quenching or oxidation of Fe(II) to Fe(III), further impairing its cyclic performance, degrading its efficiency, and increasing energy consumption [28,29]. These competing reactions highlight a fundamental challenge in HEF systems: balancing H₂O₂ generation with efficient Fe(III)/Fe(II) cycling [30,31].

Thus, designing high-performance iron-based heterogeneous catalysts, particularly recoverable iron-based cathode catalysts, represents a pivotal challenge for the successful large-scale application of HEF technology. While significant efforts have been devoted to synthesizing iron-based cathode catalysts [32], a comprehensive understanding of the electron-transfer mechanisms governing Fe(III)/Fe(II) conversion—specifically, maximizing electron delivery to accessible Fe active sites—remains elusive. Moreover, the degradation mechanisms involving radical and non-radical pathways in iron-based HEF systems still deserve more exploration [33,34]. A deeper mechanistic understanding of electron transfer and Fe(III)/Fe(II) cycling is essential to advance HEF technology.

This review aims to consolidate the current knowledge and guide future research by providing a comprehensive summary of the structural characteristics, compositional design, catalytic performance, and electron transfer mechanisms governing Fe(III)/Fe(II) redox cycling in iron-based cathode catalysts, along with advanced strategies developed to enhance their regeneration efficiency in HEF systems. A special emphasis is placed on radical and non-radical degradation pathways of EOCs in HEF systems. Specifically, this study presents a systematic classification and critical analysis of state-of-the-art iron-based cathode catalysts for HEF applications, including metallic iron and iron alloys, iron oxides, iron-carbon/nitrogen-doped carbon composites, iron sulfides/phosphides, and iron-based MOFs, with a particular analysis of their electron transfer mechanisms. Finally, this work also identifies current technological challenges and proposes future research directions aimed at improving the practical applicability and environmental relevance of HEF technology for advanced oxidative wastewater treatment, focusing on key aspects such as catalyst stability under prolonged operation, pH adaptability in complex matrices, and scalability for industrial implementation. The insights presented herein not only consolidate the current understanding of iron-based electro-Fenton systems but also provide a strategic framework for the rational design of next-generation heterogeneous catalysts with enhanced performance and broader applicability in addressing emerging water pollution challenges.

2. Main Types of Iron-Based Cathode Catalysts

The iron-based cathode catalyst serves as the core component of the HEF process, whose design and performance critically determine both Fe(III)/Fe(II) redox cycling efficiency and pollutant degradation effectiveness. Five distinct types of iron-based cathode catalysts are systematically categorized and discussed below.

2.1. Zero-Valent Iron and Iron-Based Alloy

Zero-valent iron nanoparticles (nZVI) exhibit exceptional catalytic performance in HEF systems due to their superior redox activity and high specific surface area [35,36]. The catalytic mechanism involves three interconnected pathways: (1) nZVI undergoes Fe(II) via micro-electrolysis at the cathode surface (Equation (1)), providing the active iron source for subsequent Fenton reactions; (2) the electrochemically generated H₂O₂ reacts with Fe(II) to produce ^•OH (Equation (2)), which drives efficient pollutant degradation; and (3) the reaction-derived Fe(III) is reduced back to Fe(II) by nZVI (Equation (3)), establishing a self-perpetuating catalytic cycle [37,38]. Despite these advantages, nZVI exhibit a pronounced tendency toward surface passivation during HEF operation [39]. The formed oxide layer significantly compromises electron transfer and constrains Fe(II) availability, consequently inducing nonproductive H₂O₂ decomposition (Equation (4)) [40]. To overcome this limitation, Ma et al. developed an innovative electrode design incorporating MOF-derived nZVI embedded in a conductive carbon matrix via electrospinning (Figure 1a) [41]. Their experimental results confirmed the complete mineralization of secnidazole (SCZ) within 60 min accompanied by consistent performance retention over eight operational cycles (Figure 1b,c). In this system, dual protective mechanisms are incorporated: the nZVI cores are shielded from oxidation by the surrounding graphitic carbon, while electron transfer is facilitated through the conductive network (Equation (5)). Notably, the system’s efficacy relies on a precise equilibrium between electron supply and consumption, where either excess H₂O₂ or insufficient Fe(II) leads to radical quenching side reactions [42]. Through this carbon encapsulation strategy, two key challenges are simultaneously addressed: the nZVI core is preserved against oxidation, and efficient electron transfer is facilitated. As a result, continuous Fe(II) replenishment is ensured, enabling sustained radical generation and stable pollutant degradation.

Fe⁰ + H₂O₂ → Fe(II) + 2H₂O

(1)

Fe(II) + H₂O₂ → Fe(III) + ^•OH + OH⁻

(2)

Fe⁰ + 2Fe(III) → 3Fe(II)

(3)

2H₂O₂ → O₂ + 2H₂O

(4)

Fe(III) + e → Fe(II)

(5)

Recent advancements have focused on the incorporation of bimetallic alloys to improve electron transfer efficiency [43]. Comparative studies reveal that iron-based alloy catalysts exhibit significantly enhanced performance relative to monometallic iron catalysts, which can be attributed to their unique physicochemical properties and synergistic intermetallic effects [44]. The strategic incorporation of a secondary metal element leads to the formation of bimetallic active centers that effectively modify the electronic structure of the material [45]. This electronic modulation, driven by electronegativity differences between the constituent metals, facilitates interfacial electron transfer while optimizing Fe(II) species regeneration efficiency [30,46]. As a result, iron-based alloy catalysts demonstrate superior catalytic activity and operational durability compared to their monometallic counterparts.

A prominent illustration of this approach is the two-dimensional (2D) CoFe-LDH/CF electrode developed by He et al. for HEF applications [47]. In this system, bimetallic atoms function as efficient electron donors, simultaneously activating H₂O₂ for ^•OH generation and promoting charge transfer during redox reactions; a schematic illustration of the generation of ^•OH mechanism on the surface of CoFe-LDH is shown in Figure 2a. Complementing these findings, Chen et al. established that cobalt doping precisely tunes the electronic structure of iron catalytic sites [48]. The results of electrochemical impedance spectroscopy (EIS) further indicated that the highly dispersed FeCo nanoalloy in mesochannels had the smallest Nyquist plot diameter, confirming a faster electron transfer for ORR. The schematic illustration of O₂ to ^•OH for enhanced degradation performance over FeCo nanoalloy cathode in HEF process is shown in Figure 2b. Further progress was achieved by Zhu et al., who engineered a composite architecture consisting of an alloy and bimetallic oxide (CoFe/CoFe₂O₄ supported on N-doped graphene as a cathode) [49]. This design enables efficient interfacial electron transfer and sustains stable Co/Fe redox cycling, accomplishing complete bisphenol A (BPA) removal within 90 min along with exceptional stability over 10 cycles. At the CoFe/CoFe₂O₄ heterointerface, the electron transfer mechanism involves a strongly coupled interface, where self-driven electron flow minimizes charge transfer resistance and facilitates rapid electron migration, which was proved by electrical equivalent circuit fitting. This synergistic interaction promotes concurrent generation of Fe(II) and Co(II) species, as well as efficient Fe(III)/Fe(II) valence cycling, thereby improving catalytic performance and electrode durability. A schematic diagram of proposed mechanism by the CoFe/CoFe₂O₄@NGF cathode is shown in Figure 2c.

The cooperative effects between different metals in these iron-based alloys extend beyond electronic structure optimization to include enhanced carrier density and improved conductivity. Particularly noteworthy is the exceptional 2e-ORR performance exhibited by Ni-based sites, which stems from their optimal *OOH binding energy and facilitated *HOOH detachment, yielding nearly 100% H₂O₂ selectivity [45]. Among various iron-based alloys, Fe-Ni combinations display outstanding electrocatalytic activity [50]. The incorporation of Ni proves especially effective in augmenting the 2e selectivity of Fe-based cathodes for ORR. Demonstrating this principle, Zhang et al. fabricated an innovative Fe-doped β-Ni(OH)₂ nanosheet/nickel foam (FeNi-OH/NF) cathode for HEF systems [51]. Figure 2d shows the stable structures of *OOH and H₂O₂ adsorbed on β-Ni(OH)₂ and FeNi-OH; this result demonstrated that FeNi-OH on NF not only improves H₂O₂ adsorption but also expedites Fe(III)/Fe(II) cycling, thereby enhancing the reaction between H₂O₂ and Fe(II) to produce ^•OH for efficient phenol degradation.

Figure 2. (a) Schematic illustration of the generation of the ^•OH mechanism on the surface of CoFe-LDH [47]; (b) schematic illustration of O₂ to ^•OH for enhanced degradation performance over Fe_0.5Co_0.5@OMC cathode in HEF process [48]; (c) schematic diagram of proposed mechanism by the CoFe/CoFe₂O₄@NGF cathode [49]; (d) stable structures of *OOH and H₂O₂ adsorbed on β-Ni(OH)₂ and FeNi-OH [51].

Figure 2. (a) Schematic illustration of the generation of the ^•OH mechanism on the surface of CoFe-LDH [47]; (b) schematic illustration of O₂ to ^•OH for enhanced degradation performance over Fe_0.5Co_0.5@OMC cathode in HEF process [48]; (c) schematic diagram of proposed mechanism by the CoFe/CoFe₂O₄@NGF cathode [49]; (d) stable structures of *OOH and H₂O₂ adsorbed on β-Ni(OH)₂ and FeNi-OH [51].

2.2. Iron Oxides

Iron oxide catalysts (e.g., Fe₂O₃, Fe₃O₄, and FeOOH) have garnered significant attention for HEF applications owing to their economic viability and environmental compatibility [52]. Among these candidates, Fe₂O₃ has emerged as a particularly promising catalyst due to its stable crystalline framework and abundant surface active sites, with catalytic performance primarily governed by electron transfer efficiency and Fe(III)/Fe(II) cycling kinetics [53]. In a representative study, Xu et al. fabricated a foam iron-derived electrode via a facile synthesis method (Figure 3a) [54]. The three-dimensional (3D) porous architecture and high conductivity of the iron foam substrate were leveraged to establish in situ-formed Fe₂O₃ nanosheets with direct chemical bonding to the substrate, thereby minimizing interfacial resistance and accelerating electron transfer, which was proved by the Nyquist diagram of the iron foam-derived electrode. This configuration enabled sustained reduction of Fe(III) to catalytically active Fe(II) at the cathode, ensuring persistent activity in the HEF system, and the reaction mechanism of the iron foam-derived electrode is shown in Figure 3b. Achieving a uniform dispersion of iron oxides while maintaining catalyst stability remains a critical challenge in HEF processes. To address this, the core/shell configuration has been widely implemented as an effective strategy, wherein the spatial confinement effect of the outer shell mitigates aggregation and dissolution of encapsulated iron species during catalysis, concurrently achieving enhanced electrocatalytic activity and operational stability [55,56]. For instance, Lin et al. developed a Fe@Fe₂O₃ core-shell nanowire-loaded etched carbon felt cathode (Fe@Fe₂O₃/ECF) [57]. The designed Fe⁰/Fe₂O₃ core-shell structure was shown to facilitate electron transfer while promoting both Fe(III)/Fe(II) cycling and H₂O₂ activation. The underlying mechanism involves continuous electron supply from the Fe⁰ core via micro-electrolysis, enabling the reduction of Fe(III) to Fe(II) in the Fe₂O₃ shell, while the ECF substrate mediates two-electron oxygen reduction for H₂O₂ generation. This synergistic design conferred exceptional stability and reproducibility during 240 min of continuous operation, along with highly efficient BPA degradation. Figure 3c shows the schematic diagram of the proposed mechanism for the Fe@Fe₂O₃/ECF electrode.

Iron oxyhydroxide (FeOOH) catalysts have demonstrated remarkable efficacy in HEF systems [58]. As evidenced by Zhang et al., FeOOH/activated carbon (AC) composites exhibited an effective decolorization of organic pollutants within 240 min via combined adsorption and Fenton oxidation [59]. Building on this, Wang et al. engineered an advanced HEF cathode by uniformly dispersing FeOOH within a graphene aerogel matrix [60]. Compared to conventional AC, the layered architecture of the aerogel provides ample reaction space for ^•OH-mediated degradation, while its open porous structure and enhanced conductivity collectively improve charge transfer efficiency, enabling efficient sulfamethoxazole (SMX) degradation and mineralization. The schematic diagram of proposed mechanism by the γ-FeOOH graphene polyacrylamide carbonized aerogel cathode is shown in Figure 4a. Through an alternative approach, Lv et al. constructed FeOOH nanosheet-modified carbon felt (FeOOH/CF) cathodes for enhanced sulfamerazine (SMR) removal [61]. The effect of FeOOH loading on SMR removal is shown in Figure 4b. Strong interfacial interactions between FeOOH and CF were confirmed to significantly enhance electron transfer efficiency and operational stability. Specifically, Fe-O-C interfacial bonding was demonstrated to facilitate electron transfer, thereby promoting favorable iron redox transitions in FeOOH. This improved electron mobility effectively activated in situ-generated H₂O₂, yielding abundant ^•OH radicals for efficient pollutant degradation, and its possible catalytic mechanism is shown in Figure 4c.

The inverse spinel structure of magnetite (Fe₃O₄) confers it unique catalytic properties, with octahedral sites occupied by both Fe(II) and Fe(III), enabling facile redox transitions within a stable crystalline matrix [62]. This distinctive feature has been exploited for advanced cathode materials in HEF systems. High-resolution spectroscopic analyses by Dong et al. revealed an intrinsic electron transfer mechanism through Fe(II)-O-Fe(III) bridging configurations, which sustains continuous surface Fe(II) regeneration [63]. However, prolonged operation was observed to induce a phase transformation to γ-Fe₂O₃, accompanied by gradual activity loss (Figure 5a). To circumvent this limitation, Chen et al. designed an innovative Fe₃O₄ core-shell structure integrated with NF, and the preparation process of the composite catalysts as shown in Figure 5b, which effectively overcomes the pH constraints of conventional Fenton systems, achieving 72.8% salicylic acid (SA) degradation within 120 min under neutral conditions [64]. The 3D NF substrate functions as an efficient electron conduction network, significantly reducing charge transfer resistance and facilitating rapid electron transfer. Notably, the synergistic interaction between Ni(II)/Ni(III) and Fe(III)/Fe(II) redox couples enhances both iron cycling efficiency and overall electron transfer kinetics. This design maintains exceptional catalytic performance at neutral pH while exhibiting remarkable stability, and a schematic illustration of the enhanced HEF mechanism is shown in Figure 5c. Furthermore, the inherent magnetism of Fe₃O₄ enables facile magnetic recovery, highlighting its practical potential for wastewater treatment.

2.3. Iron-Carbon/Nitrogen-Doped Carbon Composites

The strategic incorporation of iron species into carbonaceous matrices (e.g., graphene, carbon nanotubes, activated carbon) has been shown to significantly enhance both electrical conductivity and catalytic activity [65,66]. In HEF applications, iron-carbon composites exhibit superior catalytic performance, with electron transfer mechanisms primarily determined by the intrinsic conductivity of the carbon framework, accessibility of iron active sites, and synergistic interfacial interactions between metallic and carbon phases [67].

Recent investigations have elucidated that mesoporous carbon substrates promote efficient 2e-ORR for in situ H₂O₂ generation, mediated by surface oxygen-containing groups and structural defects [68]. As exemplified by the FeNi-CA composite, the porous architecture not only confines iron-based nanoparticles but also provides a high surface area and interconnected channels for pollutant diffusion [50]. This hierarchical pore structure optimizes both oxygen transport and pollutant accessibility to active sites, thereby enhancing the degradation kinetics. In a parallel study, Cao et al. developed a Fe₃O₄/Fe⁰/Fe₃C-2-600 catalytic electrode that achieved 97.7% tetracycline (TC) degradation within 12 min while maintaining >90% efficiency after 12 operational cycles [69]. Density functional theory (DFT) calculations validated the enhanced electron transfer pathways, with the comparative Bader charge analysis in Figure 6a,b revealing that Fe⁰ incorporation induces significant electronic redistribution. This redistribution promotes electron transfer toward Fe(III) species, thereby accelerating the Fe(III)/Fe(II) redox cycle, and EIS was used to evaluate the charge transfer resistance of the catalyst during the reaction. Furthermore, Fe⁰ enhances interfacial coupling between graphitic layers and the catalyst core. Charge density difference mapping demonstrates that while pristine Fe₃O₄/C interfaces exhibit limited electron transfer, Fe⁰ introduction creates an internal electric field that mediates efficient electron transfer between core and graphitic layers (Figure 6c,d). These transferred electrons subsequently participate in Fenton reactions, reducing H₂O₂ to generate ^•OH with high oxidative potential for pollutant degradation. The proposed catalytic mechanism of Fe₃O₄/Fe⁰/Fe₃C dual-cathode HEF systems is shown in Figure 6e. In a complementary approach, Chen et al. employed a NaCl-template pyrolysis method to synthesize Fe₃O₄@three-dimensional graphene nanocomposites (Fe₃O₄@3D-GNs) containing abundant oxygen functional groups as HEF cathodes [70]. The carbon matrix can serve as a support for iron or iron oxides to facilitate electron transfer. Specifically, the 3D graphene network reduces charge transfer resistance, forms conjugated Fe-O-C bonds and serves as an effective support matrix for iron oxide nanoparticles, enabling efficient carbamazepine (CBZ) degradation (92% removal in 60 min) with exceptional stability.

Nitrogen-doped carbon materials serve as crucial components for modulating electron transfer in iron-carbon composite HEF catalysts [71,72]. The incorporation of nitrogen atoms (electronegativity = 3.04) induces significant modifications to the electronic structure of the carbon matrix, enhancing both electron mobility and electron donation to π-conjugated carbon networks [73]. Specifically, pyridinic N functions as an electron donor, thereby increasing the Lewis basicity of adjacent carbon atoms and promoting O₂ adsorption and *OOH intermediate formation. Similarly, graphitic N enhances the conductivity of the carbon framework, facilitating electron transfer between the carbon surface and O₂. These synergistic effects collectively contribute to efficient H₂O₂ generation [74,75]. In the Fe/N co-doped carbon electrodes developed by Yin et al., pyridinic N and pyrrolic N were shown to facilitate Fe(III) reduction to Fe(II), thereby enhancing the Fe(III)/Fe(II) redox cycle [76]. Furthermore, nitrogen species donate unpaired electrons to Fe-N_x active sites, significantly improving electron transfer efficiency, which was proved by the low charge transfer resistance from the EIS spectra. The construction of a 3D graphite network reduces charge transfer resistance while promoting electron migration to Fe active sites. Xiao et al. engineered a N-doped graphitic-carbon-coated iron nitride composite dispersed in a N-doped carbon framework (Fe₃N@NG/NC) (Figure 6f), which demonstrated exceptional HEF performance, achieving 97% Rhodamine B (RhB) removal within 120 min at pH 5 with minimal iron leaching [77]. Notably, NG encapsulation forms a core-shell structure around Fe₃N nanoparticles, while the intimate contact between the NC framework and graphitic-carbon-coated Fe₃N enhances electron transfer within the Fe₃N@NG/NC composite during electrochemical processes. The possible HEF degradation mechanism for RhB with Fe₃N@NG/NC as the cathode is shown in Figure 6g. These nitrogen-doping strategies effectively optimize electron transfer between iron active sites and the carbon matrix, thereby substantially improving overall catalytic performance. Overall, these material design strategies have addressed the key limitations of HEF systems, and the enhanced performance stems from the optimal integration of conductive networks, nanostructured active sites, and rationally designed interfaces.

Figure 6. Bader charge distributions of the (a) Fe₃O₄ cluster and (b) Fe₃O₄-Fe cluster; the differential charge density of the (c) Fe₃O₄ cluster/C and (d) Fe₃O₄-Fe cluster/C; (e) the proposed catalytic mechanism of the Fe₃O₄/Fe⁰/Fe₃C dual-cathode HEF systems [69]; (f) a schematic diagram of the synthesis procedure used for Fe₃N@NG/NC; (g) the possible HEF degradation mechanism for RhB with Fe₃N@NG/NC/GF as the cathode [77].

Figure 6. Bader charge distributions of the (a) Fe₃O₄ cluster and (b) Fe₃O₄-Fe cluster; the differential charge density of the (c) Fe₃O₄ cluster/C and (d) Fe₃O₄-Fe cluster/C; (e) the proposed catalytic mechanism of the Fe₃O₄/Fe⁰/Fe₃C dual-cathode HEF systems [69]; (f) a schematic diagram of the synthesis procedure used for Fe₃N@NG/NC; (g) the possible HEF degradation mechanism for RhB with Fe₃N@NG/NC/GF as the cathode [77].

2.4. Iron Sulfides/Phosphides

Pyrite (FeS₂), as the most abundant iron sulfide mineral, demonstrates exceptional catalytic properties in HEF systems, attributable to its unique surface oxidation states and abundant Lewis acid sites [78]. These characteristics collectively facilitate efficient H₂O₂ decomposition into ^•OH while simultaneously promoting H₂O₂ adsorption and subsequent reaction with ferrous iron. As evidenced by Yu et al., hydrothermally synthesized FeS₂ effectively catalyzes H₂O₂ activation through surface sulfur vacancies and Fe(III)/Fe(II) redox cycling, achieving 95% diclofenac (DCF) degradation within 90 min [79]. Notably, the S^2–/S₂^2– redox couple further enhances Fe(III) reduction, significantly improving Fe(II) regeneration efficiency, and a schematic diagram of the reaction of DCF in FeS₂-HEF system is shown in Figure 7a. In a related study, Cui et al. developed a novel FeS₂/CF composite cathode through rational structural engineering, wherein uniformly dispersed FeS₂ nanoparticles were immobilized on the CF substrate as a stable iron source [80]. This configuration exhibited exceptional catalytic performance in the HEF degradation of DCF, with the enhanced activity originating from two key factors: (1) Fe-S covalent bonding significantly reduces charge transfer resistance, and (2) sulfur vacancies function as electron traps to lower the work function, collectively optimizing interfacial electron transfer. Furthermore, the S^2–/S₂^2– redox couple facilitates Fe(III) reduction by lowering the reduction potential, thereby accelerating Fe(II) regeneration through dual electron transfer pathways involving both direct Fe(III)/Fe(II) cycling and indirect sulfur-mediated redox shuttling. These findings demonstrate that iron-sulfur composites strategically leverage sulfur’s redox mediation and electronic modulation capabilities to effectively address two critical in HEF challenges of inefficient iron cycling and sluggish H₂O₂ activation.

Extensive research has established phosphorus doping as an effective strategy for optimizing the electronic structure and catalytic performance of iron-based HEF catalysts [81,82]. The incorporation of phosphorus atoms induces significant electronic modifications, transforming them into electron-rich Pδ-species (0 < δ < 1) that serve dual functions as both efficient electron donors for Fe(III) reduction and additional active sites for reactive species generation. More importantly, strong P-Fe bonding leads to the formation of iron phosphides (Fe_xP) with metallic-like properties, including high electron mobility, exceptional acid resistance, and stability. The phosphorus-induced conductive network further enhances electron transfer rates and ORR efficiency. Wu et al. demonstrated these effects through their development of a FeP@ECC cathode fabricated via a one-step low-temperature phosphorization strategy, which achieved outstanding performance metrics: 97.5% SMZ degradation within 60 min and >90% activity retention after 15 operational cycles [83]. The superior activity stems from phosphorus-mediated electronic engineering, which lowers the Fe(III)/Fe(II) redox potential and decreases the H₂O₂ activation barrier. The EIS test was carried out to evaluate the electron transfer ability of the FeP@ECC electrode, the smallest arc diameters and the lowest curve slope illustrating the smallest charge transfer resistance, meaning that the interfacial charge transfer rate is faster. The proposed mechanism for SMZ degradation by the FeP@ECC-EF system is shown in Figure 7b. Due to its higher electronegativity (P: 2.19 vs. Fe: 1.83), phosphorus promotes electron delocalization at Fe centers, enhancing their reducibility. Additionally, p-d orbital hybridization between P and Fe optimizes the d-band center position of iron, further accelerating Fe(III) reduction and iron cycling efficiency. Collectively, Fe_xP catalysts achieve highly efficient iron cycling via a “P-Fe electron bridge” system, where interfacial charge transfer at the P-Fe-C junction serves as the fundamental mechanism [82].

Figure 7. (a) Schematic diagram of the reaction of DCF in a pyrite-EF system [79]; (b) the proposed mechanism for SMZ degradation by the FeP@ECC-EF system [83].

Figure 7. (a) Schematic diagram of the reaction of DCF in a pyrite-EF system [79]; (b) the proposed mechanism for SMZ degradation by the FeP@ECC-EF system [83].

2.5. Iron-Based MOFs

MOFs have emerged as a transformative class of electrocatalytic materials due to their exceptional surface area, tunable porosity, and structural robustness [84]. Specifically, iron-based MOFs, which feature iron ions as coordination centers bridged by organic ligands to construct well-defined architectures, have shown particular promise for HEF applications because of their structural versatility and abundant active sites that promote efficient electron transfer and pollutant degradation [85].

Recent advances have demonstrated the successful integration of iron-MOFs as high-performance HEF catalysts. For instance, Zhu et al. developed a binder-free and self-supported MOF/FeOOH heterojunction cathode specifically designed to enhance Fe(III) reduction [86]. Through an innovative in situ growth strategy, the researchers fabricated an FeOOH layer on titanium foam as both iron source and conductive substrate for subsequent NH₂-MIL-88B(Fe) synthesis (Figure 8a). This hierarchical design establishes an internal electron transfer channel that enables direct electron donation to iron centers within the MOF, achieving unprecedented Fe(III) reduction efficiency, and a schematic illustration of the MOF/FeOOH heterojunction is shown in Figure 8b. Complementary DFT calculations and advanced spectroscopic characterization confirmed that the interfacial electric field optimizes electronic structures, promoting directional electron transfer from FeOOH to NH₂-MIL-88B(Fe), and the optimized H₂O₂ and adsorption configuration over the MOF and MOF/FeOOH heterojunction is shown in Figure 8c. Moreover, the heterojunction architecture significantly enhances electron transfer from iron centers to H₂O₂, thereby amplifying catalytic activity. The possible mechanism of the MOF/FeOOH heterojunction-catalyzed HEF processes is shown in Figure 8d. Further advancing this field, Yang et al. designed a 2D electroresponsive ferrocene MOF (ER-Fc-MOF) electrode capable of direct electron transfer through continuous Fc⁺/Fe(III)-to-Fc/Fe(II) cycling within its sandwich structure [87]. This innovative system demonstrates broad operational versatility, effectively degrading SMX across pH 3–9 even in complex water matrices containing competing ions. Complementing these findings, Pei et al. have elucidated the critical influence of organic ligand engineering on Fe-MOF performance [88]. Through systematic modifications of MIL-101(Fe) ligands, this work demonstrated that tailored electronic properties can precisely modulate iron center electronic states, thereby optimizing H₂O₂ activation kinetics and ^•OH generation rates. Notably, electron-donating ligands were found to lower the activation barrier for Fe(III)/Fe(II) cycling, while π-conjugated systems enhance interfacial charge transfer. Collectively, these studies collectively highlight the pivotal role of Fe-MOFs in HEF systems, including the engineered interfaces that facilitate rapid electron transfer through built-in electric fields or redox-active linkers and ligand-mediated tuning of iron electronic states to optimize reactive oxygen species production.

3. Reaction Pathways

The catalytic mechanisms of iron-based cathode catalysts in HEF systems are governed by complex reaction pathways involving both radical and non-radical species [89]. A fundamental understanding of these activation mechanisms is paramount for elucidating contaminant degradation processes and optimizing system performance. The predominance of radical versus non-radical pathways is critically dependent on catalyst structural characteristics and operational conditions, necessitating comprehensive investigation of both mechanisms to advance HEF technology [90,91].

3.1. Radical Pathways

In HEF systems, H₂O₂ acts as the primary oxidant, undergoing catalytic decomposition to generate ^•OH, which are the predominant reactive species responsible for organic pollutant mineralization. Iron-based catalysts significantly enhance this process through efficient H₂O₂ activation and sustained ^•OH production. As demonstrated by Luo et al., Cu-doped Fe@Fe₂O₃ core-shell nanoparticles deposited on Ni foam (CFF/CNT composite cathode) exhibited exceptional ^•OH generation capacity [92]. As shown in Figure 9a, radical quenching experiment results confirmed that ^•OH was the leading effective and active species during the HEF process. Subsequently, ^•OH, as the major active species, attacked TC molecules, leading to their gradual degradation and complete mineralization to H₂O, CO₂, and inorganic ions. The schematic diagram of TC removal mechanism in the HEF system with the CFF/CNT composite cathode is shown in Figure 9b, consistent with established HEF reaction mechanisms. To elucidate the electron transfer mechanisms underlying the degradation process, Figure 9c presents the proposed reaction pathways for TC and 14 representative intermediates during the HEF process. A detailed structural analysis of intermediate products revealed that functional groups with high electron density served as the primary target for radical attack, initiating a series of transformation of the main intermediates. These identified intermediates subsequently underwent progressive fragmentation through multiple oxidation steps, ultimately mineralizing into inorganic end-products including CO₂, H₂O, and various ionic species. This degradation pathway was facilitated by the efficient electron transfer properties of the CFF/CNT composite cathode, which promoted the continuous generation of ^•OH while maintaining stable catalytic performance throughout the reaction process. As the HEF reaction proceeded, the toxicity of the intermediate products was evaluated by the ECOCAR software (Version 2.0). The toxicity profile typically exhibits a decreasing trend as oxidation progresses, corresponding to the sequential transformation of complex organic structures into simpler, less harmful compounds. To reduce the environmental risks posed by intermediates in the degradation process, it is necessary to extend the reaction time to ensure more complete TC mineralization [69].

3.2. Non-Radical Pathways

Although radical species (e.g., ^•OH) exhibit exceptional reactivity, their effectiveness is frequently compromised in complex wastewater matrices due to rapid scavenging by background constituents, thereby diminishing their degradation selectivity. In comparison, non-radical oxidants such as singlet oxygen (¹O₂) demonstrate superior environmental persistence and reduced susceptibility to matrix interference, rendering them particularly advantageous for treating challenging waste streams [93]. The formation of ¹O₂ primarily occurs through the recombination of superoxide radicals (^•O₂⁻) and hydroperoxyl radicals (^•HO₂), as exemplified by the mechanistic studies of Chen et al. [70]. The researchers engineered a Fe₃O₄@3D-GNs that selectively catalyzes the 2e-ORR to generate H₂O₂, which is subsequently transformed into ¹O₂ rather than conventional ^•OH radicals, electron paramagnetic resonance (EPR) spectra and quenching experiments confirm this result (Figure 10a,b). DFT calculations elucidated the mechanism (Figure 10c). The elucidated reaction sequence was as follows: H₂O₂ undergoes O-H bond cleavage at Fe-OH sites, yielding ^•HO₂, then recombines to produce ¹O₂ via its subsequent transformations (Equations (6)–(8)). This catalytic pathway highlights the distinctive advantage of non-radical mechanisms in overcoming the limitations associated with conventional radical-based oxidation processes.

H₂O₂ + ^•OH → H₂O + ^•O₂H

(6)

^•OH + ^•O₂H → H₂O + ¹O₂

(7)

2^•O₂H → H₂O₂ + ¹O₂

(8)

In practical applications, radical and non-radical pathways frequently coexist, complicating mechanistic interpretation. For instance, Qi et al. developed CuFe₂O₄/Cu₂O/Cu@etched graphite felt (CuFe₂O₄/Cu₂O/Cu@EGF) cathodes that facilitated concurrent ^•OH, ^•O₂⁻, and ¹O₂ production, with ^•OH identified as the primary reactive species through combined experimental and theoretical analyses (Figure 11a,b) [94]. Similarly, Li et al. engineered a trimetallic cathode of CoFe₂O₄@MoS₂@graphite felt (CoFeMo@GF) that synergistically enhanced both radical (^•OH/^•O₂⁻) and non-radical (¹O₂) pathways, as confirmed by EPR spectroscopy (Figure 12a–c) [95]. Furthermore, the role of light in the HEF process was also explored. Under illumination, photogenerated electrons are excited and migrate to the conduction band, while photogenerated holes are formed at the valence band position. Photogenerated electrons are conducive to the generation of ^•OH radicals, and the electrons photogenerated from the photoanode will be utilized to in situ generate H₂O₂ via a two-electron reduction pathway. Figure 12d elucidates the reaction mechanism of ROS generation and organics degradation in the CoFeMo@GF system, demonstrating simultaneous enhancement of both radical and non-radical pathways that enable near-complete pollutant mineralization while maintaining excellent cathode stability. These findings were further corroborated by Liu et al. through frontier orbital theory calculations, collectively demonstrating the complex interplay between multiple degradation mechanisms in iron-based EF systems (Figure 13) [96].

4. Conclusions and Perspectives

This critical review synthesizes fundamental insights and recent breakthroughs in electron transfer mechanisms of iron-based cathode catalysts for EOC degradation in HEF systems. The analysis establishes that iron-based catalysts hold an unparalleled potential for HEF applications due to their exceptional electron transfer capacity and rich diversity of redox-active sites, with catalytic performance being fundamentally governed by electron transfer efficiency. Through systematic structural and compositional optimization, substantial enhancements in electron transfer kinetics can be realized, thereby improving both Fe(III)/Fe(II) cycling and overall catalytic efficiency. We have provided a comprehensive evaluation of five principal categories of iron-based cathode catalysts, including metallic iron and iron alloys, iron oxides, iron-carbon/nitrogen-doped carbon composites, iron sulfides/phosphides, and iron-based MOFs, with particular emphasis on their structural design, catalytic performance, and electron transfer mechanisms. Despite sharing iron as the common active center, various iron-based catalysts exhibit substantial variations in electron conduction capacity, active site distribution, stability, and pH adaptability, which collectively determine their degradation efficiency for EOCs. Metallic iron and iron alloys demonstrate superior metallic conductivity, enabling direct facilitation of electron transfer processes. Nevertheless, these systems are particularly susceptible to surface passivation through Fe₂O₃ layer formation, leading to a marked decline in Fe(III)/Fe(II) cycling rates. In comparison, iron oxide catalysts operate through semiconductor-mediated electron transfer mechanisms. However, their constrained conduction band structures impose limitations on electron mobility, necessitating carbon-based modifications to achieve enhanced conductivity. Iron-carbon composite materials, particularly nitrogen-doped variants, display exceptional electron transfer efficiency, attributed to their highly conductive carbon matrices coupled with well-dispersed Fe-N_x active sites. Notably, nitrogen doping serves a dual function: not only does it optimize the electronic configuration of iron active centers, but it also significantly improves H₂O₂ activation capabilities via metal-support interactions. This synergistic effect yields an enhancement in electron transfer efficiency relative to pure iron systems while simultaneously achieving an optimal balance between catalytic activity and operational stability. Iron sulfide and phosphide catalysts leverage their distinctive metal-nonmetal bonding characteristics to effectively promote Fe(III)/Fe(II) redox cycling. Conversely, iron-based MOF materials, despite offering precisely tunable coordination environments for selective pollutant adsorption, suffer from inherent conductivity limitations. Consequently, thermal conversion to iron-carbon composites is typically required to attain satisfactory electron transfer rates. Furthermore, this review elucidates contaminant degradation pathways mediated through both radical (^•OH) and non-radical (¹O₂) species, along with their synergistic effects in pollutant mineralization.

Notwithstanding these advancements, several pivotal challenges demand urgent attention in future research endeavors:

(1) Catalyst stability limitations: Progressive deactivation through active site passivation or iron leaching during extended operation remains a critical barrier. Innovative solutions such as protective nanoscale coatings, stable coordination environments, or self-regenerative architectures must be developed to ensure practical viability in complex wastewater matrices.

(2) Electron transfer optimization: While the current research has mapped fundamental electron transfer pathways, strategic enhancements in conductive networks (e.g., graphene hybridization) and electronic structure modulation (e.g., heteroatom doping) are required to achieve maximal Fe(III)/Fe(II) cycling rates and H₂O₂ activation efficiency.

(3) pH adaptability constraints: The predominant pH limitation (pH 2–4) of conventional systems necessitates the development of pH-universal catalysts via buffer-functionalized ligands or multi-active-site engineering, enabling effective operation across neutral-to-alkaline conditions without activity compromise.

(4) Mechanistic interrogation: Advanced in situ characterization techniques (e.g., X-ray absorption spectroscopy, Raman) coupled with DFT calculations should be leveraged to decipher real-time electron transfer dynamics and reactive species evolution. A fundamental understanding of radical/non-radical pathway interplay will permit precise control over degradation selectivity in complex effluents.

(5) Technology translation: Large-scale validation is imperative to assess catalyst performance in authentic industrial wastewater containing multicomponent pollutants. Synergistic integration with complementary technologies (e.g., membrane filtration, bioremediation) may overcome inherent HEF limitations while optimizing energy efficiency.

Future research trajectories should focus on the rational design of iron-based catalysts that integrate high catalytic activity, long-term operational stability, economic feasibility, and broad environmental compatibility. By addressing these multidisciplinary challenges, HF technology can evolve from bench-scale innovation to practical wastewater treatment solutions, ultimately contributing to global water sustainability goals.