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New insights into electrocatalytic singlet oxygen generation for effective and selective water decontamination
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Shengtao Jianga, Mengjiao Xieb, Limin Jinb, Yifan Renb, Wentian Zhengb, Siping Jic, *, Yanbiao Liub, *
Chinese Chemical Letters | 2025, 36(5) : 110293
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Chinese Chemical Letters | 2025, 36(5): 110293
• Communication •
New insights into electrocatalytic singlet oxygen generation for effective and selective water decontamination
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Shengtao Jianga, Mengjiao Xieb, Limin Jinb, Yifan Renb, Wentian Zhengb, Siping Jic, *, Yanbiao Liub, *
Affiliations
  • a Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Onservation, School of Life Science, Taizhou University, Taizhou 318000, China
  • b College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
  • c School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, China
Published: 2025-05-15 doi: 10.1016/j.cclet.2024.110293
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Singlet oxygen (1O2), as an electrophilic oxidant, is essential for the selective water decontamination of pollutants from water. Herein, we showcase a high-performing electrocatalytic filtration system composed of carbon nanotubes functionalized with CoFe alloy nanoparticles (CoFeCNT) to selectively facilitate the electrochemical activation of O2 to 1O2. Benefiting from the prominently featured bimetal active sites of CoFeCNT, nearly complete production of 1O2 is achieved by the electrocatalytic activation of O2. Additionally, the proposed system exhibits a consistent pollutant removal efficiency > 90% in a flow-through reactor over 48 h of continuous operation without a noticeable decline in performance, highlighting the dependable stability of the system for practical applications. The flow-through configuration demonstrates a striking 8-fold enhancement in tetracycline oxidation compared to a conventional batch reactor. This work provides a molecular level understanding of the oxygen reduction reaction, showing promising potential for the selective removal of emerging organic contaminants from water.

CoFe alloy  /  Oxygen activation  /  Singlet oxygen  /  Electrocatalytic system water decontamination
Shengtao Jiang, Mengjiao Xie, Limin Jin, Yifan Ren, Wentian Zheng, Siping Ji, Yanbiao Liu. New insights into electrocatalytic singlet oxygen generation for effective and selective water decontamination[J]. Chinese Chemical Letters, 2025 , 36 (5) : 110293 - . DOI: 10.1016/j.cclet.2024.110293
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Recently, the effective and selective removal of emerging organic contaminants (EOCs) has drawn significant attention [1-4]. A notable surge in the advancement of molecular oxygen (O2) activation technology has drawn considerable interest within the realm of advanced oxidation processes (AOPs). This innovative approach can generate a variety of reactive oxygen species (ROS) [5], including free radicals (e.g., hydroxyl radicals (•OH) and sulfate radicals) and non-radical species such as singlet oxygen (1O2). 1O2 is a highly reactive non-radical ROS that offers a selective ability to oxidize (E0(1O2/H2O) = 2.17 V vs. NHE) electron-rich micropollutants (ng/L to µg/L) as well as a high tolerance to environmental factors that could interfere with oxidation [6-8]. The primary techniques for producing 1O2 in AOPs rely on the utilization of hydrogen peroxide (H2O2) and peroxymonosulfate [9,10]. Nevertheless, their practical utility is limited by their need for external chemical inputs, intrinsic limits in activation selectivity and inadequate ability to degrade micropollutants [11]. Hence, advanced strategies for the direct synthesis of 1O2 from ground state O2 are highly desirable.
The formation of 1O2 is linked to the "end-on" adsorption configuration of O2 on the metal active sites, promoting the cleavage of an M–O bond instead of an O–O bond, which also plays a pivotal role in fostering the formation of –O–O– intermediates (critical intermediates for 1O2) [12,13]. The "end-on" adsorption O2 to metal sites significantly increases the likelihood of 1O2 production by enabling the reduction or disproportionate reaction of –O–O– intermediates [14-16]. Consequently, these problems may be addressed by formulating innovative approaches to enhance the efficacy of the transformation of *OOH into 1O2 [17-19].
To this end, catalysts based on Prussian blue analogues (PBAs) that contain transition metal cations and CN− groups have been extensively pursued for the oxygen reduction reaction (ORR) because of their adjustable composition and exceptional electrochemical properties [20-23]. A crucial challenge regarding the synthesis of PBA-derived catalyst is their irreversible agglomeration and collapse. Fortunately, one-dimensional carbon nanotubes (CNT) with exceptional chemical stability are considered as a promising substrate to host a range of metallic nanoparticles [24]. Additionally, one-dimensional CNT can be converted into three-dimensional conductive networks through vacuum filtration to enable flow-through operations [25-27]. Such designs minimize the diffusional distance of reactants and expedited electrochemical reactions, thereby bolstering both the destruction of pollutants and the anti-fouling performance of the reactive filter membrane [28,29].
Herein, we present a facile strategy to functionalize CoFe alloy nanoparticles on CNT (CoFeCNT) to electrocatalytically transform O2 into 1O2 via a 3-electron pathway in a flow-through configuration. The results show that the targeted production of 1O2 is successfully attained during the electrocatalytic process through the incorporation of adjacent Co atoms at Fe active sites. The Co atoms effectively modulate the electronic structure of the resulting alloy nanoparticles, leading to a substantial enhancement in both catalytic performance and durability. Through a combination of electrochemical characterizations and experimental results, the alloying effects of CoFeCNT are shown to facilitates the donation of electrons from Co to Fe, which proves advantageous for the creation of 1O2 via electron transfers from Co sites to *H2O2. Furthermore, the system efficacy for purifying contaminated water is assessed using tetracycline (TC) as a representative organic contaminant. The essential process parameters are identified and fine-tuned to achieve optimal performance. This study offers valuable insights into the structure-performance correlation of metal-based alloy catalysts and the activation of molecular O2 for the efficient elimination of organic micropollutants via the formation of 1O2.
All chemicals and reagents were of analytical grade and used without further purification (Text S1 in Supporting information). The synthesis of the CoFeCNT nanohybrid filter started from the pyrolysis of CoFe-PBA. Details on the preparation of PBA precursor and material characterization are provided in Fig. S1 and Texts S2-S9 (Supporting information).
Fig. 1a elucidated the facile synthesis procedure of the CoFeCNT. Representative X-ray diffraction (XRD) patterns matched well the standard card (PDF #49–1568), confirming the successful fabrication of CoFeCNT with moderate crystallinity (Fig. 1b). Scanning electron microscopy (SEM) images showed that the CoFe alloy nanoparticles (diameter 200±10 nm) were randomly and homogeneously distributed on the interlaced CNT networks after the high-temperature calcination treatment (Fig. 1c). Transmission electron microscopy (TEM) was used to determine crystallinity of the CoFeCNT had a lattice spacing of 0.20 nm, indexed to the (110) facet of CoFe alloy [30], which matched the XRD results (Fig. 1d). The existence of Co and Fe in CoFeCNT was proven with energy dispersive X-ray spectroscopy (EDS) mapping of the CoFeCNT (Fig. 1e), in which the elements N, C, and O were homogeneously distributed throughout the structure. X-ray photoelectron spectroscopy (XPS) was employed to further investigate the elemental bonding status and the electronic interactions of Co and Fe. The Fe 2p XPS spectrum (Fig. 1f) showed that the binding energy of CoFeCNT was down-shifted compared to that of FeCNT, indicating a modification in the electron density of states, which could enhance the electrocatalytic activity and the adsorption of reaction intermediates [31]. In contrast, the peaks corresponding to the Co species in the XPS spectrum of CoFeCNT were shifted to a higher binding energy (Fig. 1g), indicating the transfer of charge between Fe and Co [32]. The XPS analysis revealed that charge transfer was caused by the reconstruction of electronic structure occurring with the CoFe alloying. This reconstruction could significantly affect the ORR performance of the CoFeCNT electrocatalyst [17].
The electrocatalytic ORR performance of the synthesized electrocatalysts toward O2 activation was investigated first with cyclic voltammetry (CV). Fig. 2a showed a more positive ORR cathodic peak (0.72 V vs. RHE) corresponding to CoFeCNT in an O2-saturated electrolyte (denoted as CoFeCNT/E/(O2)), providing evidence of the significant enhancement of the material for catalyzing the ORR. To better understand the underlying mechanism of the evolution of O2 molecules on the solid-liquid interface, a rotating ring-disk electrode (RRDE) measurement was conducted due to its advantages in elucidating the ORR behavior of electrocatalysts. Linear sweep voltammetry (LSV, Fig. 2b) curves of the as-synthesized electrocatalysts showed that the CoFeCNT electrocatalyst exhibited an evident positive onset potential (Eonset = 0.61 V vs. RHE) to its monometallic counterparts, revealing the synergistic effect of the bimetallic sites in terms of enhancing the activation of O2. Another important characteristic of the electrocatalysts was the selectivity of the 3e– ORR relative to the more conventional 2e− or 4e− reduction of O2 to H2O2 or H2O. To better understand this selectivity, the average electron transfer number (n) was calculated from the disk and ring current density to further assess the 3e− ORR performance using different samples as cathode materials. As presented in Fig. 2c, both FeCNT and CoCNT provided a less favorable 3e− pathway, with n values calculated to be 3.6 and 2.4, respectively. For comparison, CoFeCNT had a much greater 3e− ORR activity with an n value of ~3.0 over a potential range of 0.1–0.6 V vs. RHE, revealing that the alloy effect effectively changed the reaction path of the ORR process [33]. A pivotal characteristic of 3e− ORR process is its capacity to function independently of H2O2 while effectively removing organic pollutants [34]. The yields of H2O2 in different fluidic systems were measured with UV–vis spectroscopy. As shown in Fig. S2 (Supporting information), the concentration of accumulated H2O2 in the FeCNT/E/(O2) and CoCNT/E/(O2) system within 90 min was 16.9 and 10.1 mmol h−1 m−2, respectively. In contrast, a negligible amount of H2O2 was detected in the CoFeCNT/E/(O2) system, indicating that the possibility of a fast one-electron transfer between *H2O2 and Co sites, and there by implying the existence of the 3e− ORR.
In a typical ORR process, a multistep electron transfer pathway is involved, which can generate several intermediates, such as *O, *OH, and *OOH. To analyze the ORR process in this work, in situ Fourier-transform infrared (FTIR) spectroscopy was performed to recognize the stretching frequency corresponding to reaction intermediates formed during the 3e− ORR in the CoFeCNT/E/(O2) system. The characteristic peaks at 1276 cm−1 and 1339 cm−1 emerged and intensified (Fig. 2d), which indicated the O–O stretching mode exhibited by *O2 on the CoFeCNT surface (aligning with the end-on (Pauling model) adsorption configuration) [15,35]. Also, the intensity of *OOH continued to increase appreciably, while the band associated with the *OOH bending vibration of *HOOH at 1353 cm−1 maintained a steady presence. This suggested a prevailing dissociative pathway, in which the strong attachment of *OOH to the electrode surface disrupted the O–O bond, potentially generating ROS [34].
To reveal the mechanism of catalytic behaviors in the CoFeCNT/E/(O2) system, the generated ROS was first examined with conducting radical quenching experiments. The same concentration (50 mmol/L) of furfuryl alcohol (FFA, a scavenger for 1O2), tert–butanol (TBA, a scavenger for •OH), superoxide dismutase (SOD, a scavenger for O2•−) and catalase (a scavenger for H2O2) were introduced. The results confirmed that excessive amounts of FFA exerted a more pronounced inhibitory effects than TBA, underscoring the significance of 1O2 as the predominant ROS (Fig. 2e). Additionally, a comparable inhibitory effect was observed in the presence of SOD and catalase, implying the potential involvement of O2•− and H2O2 in the degradation of pollutants, which likely stemmed from their function as intermediates for the generation of 1O2.
Moreover, the direct observation of radicals within diverse systems is necessary to fully understand the underlying mechanism. Electron paramagnetic resonance (EPR) spectroscopy was utilized to differentiate different ROS by employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidone (TEMP) as spin-trapping agents. Prominent signals for the TEMP-1O2 adduct (with a height ratio = 1:1:1) appeared in the CoFeCNT/E/(O2) system, indicating the substantial production of 1O2. Nevertheless, no discernible EPR signals corresponding to TEMP-1O2 adducts were detected in the O2-free condition (Fig. S3 in Supporting information), implying that the generation of 1O2 is specifically linked to the 3e− ORR process. A relatively subdued peak intensity of the TEMP-1O2 adduct occurred in both the FeCNT/E/(O2) and CoCNT/E/(O2) systems, possibly indicating the comparatively poor 3e− ORR activity of the FeCNT and CoCNT electrodes (Fig. 2f). In addition, there was a notable absence of signals associated with the DMPO-O2•− and DMPO-•OH adducts in the CoFeCNT/E/(O2) system, again inferring that 1O2 served as the predominant ROS (Fig. S4 in Supporting information).
On the basis of these results, a schematic illustration outlining the catalytic activation mechanism of CoFeCNT was developed, as depicted in Fig. 3. In the process of the electroactivation of molecular O2, the O2 was preferentially adhered "end-on" onto the Fe sites on the surface of CoFeCNT to form *O2, which was facilitated by the downward adjustment of the D-band center in the electronic structure of Fe, a modification brought about by the introduction of Co [33]. Subsequently, *O2 was efficiently transformed into *H2O2 via a swift two-step process involving single-electron transfers [20]. Since the alloy effect of CoFeCNT altered the intermetallic electronic structure and enhanced the adsorption affinity of the reactants and intermediates at the catalytic sites, the *H2O2 swiftly produced 1O2 on Co sites through a fast one-electron reduction pathway [23].
The swift escalation of antibiotic pollution in the environment poses a significant concern for human health. In contrast to other ROS, 1O2 is believed to offer a new strategy for selective environmental remediation because it can preferably react with electron-rich moieties of organic molecules in complex water matrices and is less susceptible to interference from background materials. As a proof of concept, the antibiotic TC was selected as an example target contaminant to assess the electrocatalytic capabilities of the CoFeCNT filter. The TC degradation efficiency of the CoFeCNT/E/(O2) system (96%) was 1.5 times and 1.2 times higher than that of FeCNT/E/(O2) and CoCNT/E/(O2) systems, respectively (Fig. 4a), being one of the most active catalysts via 3e− ORR. Moreover, only 30% of TC was degraded with CNT alone, which was possibly due to an electrochemical adsorption effect (Fig. S5 in Supporting information). Physical and electrochemical adsorption accounted for 24% and 43% of the TC removal, respectively (Fig. 4b), indicating that the TC degradation was overwhelmingly induced by electrocatalytic reaction with the CoFeCNT filter in the presence of O2. In addition, the TC degradation in the CoFeCNT/E/(O2) system significantly decreased without O2 or current density, which was consistent with the electrochemical adsorption and physical adsorption process, respectively (Fig. 4c).
To examine the mineralization degree of TC, the change of total organic carbon (TOC) was evaluated during the degradation of TC. The CoFeCNT/E/(O2) system not only showed complete TC removal but also a large removal of TOC (60%) within 90 min (Fig. 4d). The electrocatalytic efficacy of the CoFe/CNT/E/(O2) filtration system was also compared to that of a conventional batch reactor (Fig. S6 in Supporting information). The resulting degradation efficiency in the flow-through system was significantly higher than that of traditional batch-fed reaction mode (e.g., with a TC degradation of only 13%). The significant enhancement was due to the enhanced mass transport provided by forced convection in the flow-through configuration, while the batch reactor was limited by diffusion as its primary mass transfer pathway.
The nonradical mechanism governed by 1O2 also demonstrated remarkable resistance to environmental disturbances. As illustrated in Fig. 4e, all common inorganic anions (e.g., SO42−, NO3−, Cl−, HPO4−, HCO3−) posed a negligible impact on the degradation of TC in the CoFeCNT/E/(O2) system. The CoFeCNT/E/(O2) electrocatalytic system was also effective in activating O2 over a broad pH range, underscoring the high pH tolerance of both the catalyst and 1O2 (Fig. S7 in Supporting information). Significantly, the CoFeCNT/E/(O2) system achieved consistent TC removal in a variety of water substrates (Fig. 4f, Table S1 in Supporting information), highlighting its promising prospects for practical utility. Attributed to the electron-capturing trait of 1O2, the differential efficacy of the CoFeCNT in degrading electron-deficient micropollutants (carbamazepine, and 4-nitrophenol) compared to electron-rich contaminants (TC, bisphenol A and sulfamethoxazole), suggested a discernible degradation selectivity of the CoFeCNT/E/(O2) system (Fig. S8 in Supporting information). Additional experiments were undertaken to investigate the impacts of catalyst dosage, flow rate and current density on the degradation of TC (Figs. S9 and S10 in Supporting information). Moreover, stability tests illustrated the outstanding reusability of CoFeCNT filter (Fig. S11 in Supporting information), demonstrating a consistent reactivity (> 90%) and minimal leaching of Co or Fe (< 0.01 mg/L) over a 48 h of successive runs, which is well below the standard of the World Health Organization (< 0.3 mg/L).
In summary, an intensified and sustainable strategy was proposed to fabricate fluidic CoFeCNT alloy electrodes for selectively electroactivating the reduction of O2 to 1O2, and the viability was validated with both proof-of-concept investigations and practical applications in remediation efforts. The alloying of Co with Fe facilitated the electron donation from Co to Fe, which generated an ample supply of positively charged Co atoms that served as highly efficient sites for adsorbing H2O2 during the 3e− ORR process. Simultaneously, the redistribution of charges via the cohesive Co–Fe bonds boosted the activation of *H2O2, ultimately leading to the creation of 1O2. The exceptional system efficacy was sustained in complicated water matrices and was extended to various electron-rich micropollutants. This work sheds light on the mechanisms involved in activating molecular oxygen using transition metal oxides and presents a universal design framework for electrocatalysts, enhancing the effectiveness of current high-performance methods for the electrocatalytic production of 1O2.
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Year 2025 volume 36 Issue 5
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doi: 10.1016/j.cclet.2024.110293
  • Receive Date：2024-04-21
  • Online Date：2025-10-29
  • Published：2025-05-15
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  • Received：2024-04-21
  • Revised：2024-07-01
  • Accepted：2024-07-23
Affiliations
    a Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Onservation, School of Life Science, Taizhou University, Taizhou 318000, China
    b College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
    c School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, China

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*E-mail addresses: (S. Ji), (Y. Liu).
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
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Percentage of total
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鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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