Homogeneous bismuth dopants regulate cerium oxide structure to boost hydrogen peroxide electrosynthesis via two-electron oxygen reduction

Abstract

The electrochemical synthesis of hydrogen peroxide (H₂O₂) through the two-electron oxygen reduction reaction (2e-ORR) offers a promising alternative to the traditional anthraquinone process. However, this method often suffers from sluggish kinetics. In this study, we introduce a novel bismuth-doped cerium oxide (Bi-CeO₂) composite, featuring hollow nanospheres and triangular nanoplate structures with highly dispersed Bi dopants on the CeO₂ matrix. Notably, the morphology of Bi-CeO₂ can be dynamically tuned between spheres and plates by adjusting the amounts of Bi dopants. This innovative 1%-Bi-CeO₂ catalyst exhibits an exceptional H₂O₂ selectivity of 62.3% and significantly enhanced H₂O₂ yield, reaching 1.16 mol g_cat⁻¹ h⁻¹ at 0.1 V with a high faradaic efficiency of 56.0%. Density functional theory (DFT) calculations reveal that Bi dopants effectively lower the free energy barrier for *OOH intermediate formation, thereby accelerating H₂O₂ production. Additionally, when integrated into a dual-cathode system, 1%-Bi-CeO₂ demonstrates superior performance in removing organic dyes such as rhodamine B (RhB). This work offers a groundbreaking approach for designing high-efficiency heteroatom-doped catalysts for the 2e-ORR, paving the way for more effective electrochemical systems.

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Introduction

Hydrogen peroxide (H₂O₂) is a high-value oxidant with excellent properties, notably its strong oxidizing capability and environmental friendliness, making it widely used in wastewater treatment. Currently, the conventional anthraquinone process for large-scale H₂O₂ production involves complex multistep reactions and inevitably generates organic waste. As an alternative, the electrocatalytic two-electron oxygen reduction reaction (2e-ORR) offers a green and straightforward method for H₂O₂ production.^1–5 However, achieving high electron selectivity is challenging due to the strong competition from the four-electron (4e-) ORR, which complicates the efficient generation of H₂O₂ and impacts the effectiveness of pollutant treatment. Therefore, the rational design of highly electron selective and efficient electrocatalysts for the 2e-ORR is meaningful but still challenging in this field.^6,7 While noble metal-based materials have demonstrated high activity for the 2e-ORR, their scarcity and high cost hinder practical applications.^8–11 This underscores the urgent need for highly active, stable, and cost-effective 2e-ORR catalysts to overcome current limitations.

Transition-metal compounds (TMCs), such as metal chalcogenides, metal phosphides, transition metal oxides, metal carbides, and metal nitrides, are prominent Earth-abundant nanomaterials widely used in electrocatalysis.^12–20 Among these, cerium oxide (CeO₂) stands out due to its high oxygen storage capacity and the reversible Ce⁴⁺/Ce³⁺ redox couple, making it a candidate for the two-electron oxygen reduction reaction (2e-ORR).^21,22 Despite these advantages, pristine CeO₂ suffers from poor electrical conductivity and tends to agglomerate into particles, leading to suboptimal electrocatalytic activity.^23,24 Hence, exploration of efficient strategies to rationally modulate the physical properties of CeO₂ to boost the 2e-ORR performance is significant. To date, a variety of strategies have been studied, including the design of special morphologies,²⁵ construction of heterojunctions,^26–29 heteroatom doping,^30–32etc. Based on the above analysis, it is crucial to fully utilize these strategies to design CeO₂ based catalysts with controllable physical structures to accurately regulate their inherent activity.

Herein, bismuth-doped cerium oxide (Bi-CeO₂) electrocatalysts were constructed via a simple solvothermal strategy followed by high-temperature annealing. The amounts of Bi dopants were controlled by varying the quantity of Bi raw materials. However, excessive Bi led to significant aggregation of Bi particles. The resulting Bi-CeO₂ exhibited a composite morphology of microspheres and triangular nanoplates, with the sphere diameter initially decreasing and then increasing as the Bi content increased. At a Bi atomic percentage of 1%, the Bi-CeO₂ showed the smallest sphere diameter with thin Bi-CeO₂ sheets enveloping the spheres. Electrochemical analysis revealed that the 1.0%-Bi-CeO₂ catalyst exhibited the highest two-electron oxygen reduction reaction (2e-ORR) activity, outperforming other electrocatalysts. This exceptional performance is attributed to the Bi dopants’ influence on the electronic structure and the optimized sphere/sheet architecture, which enhances electrochemical interface areas. A dual-cathode system was employed to study the degradation of organic dyes, such as rhodamine B (RhB), using in situ generated H₂O₂, achieving an impressive RhB degradation rate of over 99.7%. Additionally, a potential interaction mechanism between x%-Bi-CeO₂ and H₂O₂ was proposed. This work presents an effective approach for the precise introduction of hetero-dopants to develop highly efficient 2e-ORR electrocatalysts.

Results and discussion

The synthesis of CeO₂ and x%-Bi-CeO₂ catalysts followed the process outlined in Fig. 1. Initially, a solvothermal procedure was employed to create a sphere/plate composite morphology. Subsequently, a simple post-annealing process was used to finalize the Bi-CeO₂ catalysts. Notably, during the high-temperature annealing, some anion crosslinked products formed during the solvothermal process were oxidized, generating pores that enhance the electrochemical interfaces for the oxygen reduction reaction (ORR).

Fig. 1 Schematic illustration of the synthesis of the x%-Bi-CeO₂ catalyst.

To investigate the physical structure of the catalysts, X-ray diffraction (XRD) was performed. As depicted in Fig. 2a, the diffraction peaks at 2θ = 28.5°, 32.8°, 47.4°, and 56.3° correspond to the cubic fluorite-type CeO₂ (Fm3m space group) (PDF#75-0012), matching the (1 1 0), (2 0 0), (2 2 0), and (3 3 1) crystal planes, respectively. This indicates that the CeO₂ phase in the catalysts exhibits high crystallinity.^33,34 Additionally, there was no significant shift in the diffraction peak positions compared to pure CeO₂, suggesting that Bi doping does not substantially alter the crystal structure of CeO₂. The peaks at 2θ = 27.2°, 37.9°, and 39.6° correspond to the (0 1 2), (1 0 4), and (1 1 0) planes of rhombohedral Bi (PDF#85-1329).³⁵ This preliminary analysis indicates that the catalysts comprise both Bi and CeO₂ phases. In comparison, x%-Bi-CeO₂ (x = 0.5, 1) samples do not exhibit a distinct peak at 2θ = 27.2°, likely due to the high dispersion of Bi species within the composite.

Fig. 2 (a) XRD patterns, (b) Raman spectra, (c) high-resolution Ce 3d XPS spectra with the corresponding histogram of Ce species (d), (e) high-resolution O 1s XPS spectra with the corresponding histogram of O species (f) for CeO₂ and x%-Bi-CeO₂, and (g) high-resolution Bi 4f XPS spectra with the corresponding histogram of Bi species (h) for x%-Bi-CeO₂.

Raman scattering provides detailed information about the electronic and phonon structure of materials. CeO₂, with its cubic fluorite-type structure belonging to the Fm3m space group, features a simple vibrational structure with one infrared-active phonon of symmetry T_1μ and one Raman-active phonon of symmetry T_2g. Comparably, the 1%-Bi-CeO₂ sample exhibits a decrease in crystallite size and a broadening of the F_2g Raman band (Fig. 2b).³⁶ To probe the surface state of the catalysts, X-ray photoelectron spectroscopy (XPS) was performed. The full-range XPS survey spectrum (Fig. S1†) shows the presence of Bi, Ce, and O in the Bi-CeO₂ catalysts. High-resolution XPS spectra of Ce 3d (Fig. 2c) reveal that pure CeO₂ displays eight peaks at 882 (I), 885 (II), 888 (III), 897 (IV), 900 (V), 903 (VI), 907 (VII), and 916 eV (VIII), consistent with the previously reported literature.³⁷ The peaks at around 897, 888, and 882 eV correspond to the Ce 3d5/2 state of Ce⁴⁺, while the peaks at 916, 907, and 900 eV correspond to the Ce 3d3/2 state of Ce⁴⁺.³⁸ The small peaks at ∼885 and 903 eV, associated with the Ce³⁺ 3d5/2 and 3d3/2 states, indicate the coexistence of Ce³⁺ and Ce⁴⁺ states in both CeO₂ and Bi-doped CeO₂ catalysts. The percentages of Ce species are shown in Fig. 2d, indicating that Ce is predominantly in the +4 oxidation state. Notably, the Ce 3d peaks of 1%-Bi-CeO₂ are shifted to lower binding energies compared to other catalysts, likely due to the high electronegativity of Bi, which attracts the lone pair electrons of Ce atoms, causing a blue shift in the XPS spectrum. Thus, the presence of mixed valence of Ce³⁺/Ce⁴⁺ plays an important role in boosting the electrochemical activity.³⁹ The O 1s spectra are broad and complex due to overlapping peaks from various chemical oxygen species bound to Ce and Bi ions in the catalyst. The high-resolution O 1s spectrum (Fig. 2e) deconvolutes into four peaks at 533, 531, 530, and 529 eV, corresponding to surface-adsorbed molecular water (H₂O), surface-adsorbed hydroxyl groups or oxygen (OH⁻/O₂), surface oxygen vacancies (O₂²⁻/O⁻), and lattice oxygen (O²⁻), respectively.⁴⁰

The peak shifts in doped CeO₂ compared to pure CeO₂ are attributed to changes in the lattice oxygen environment caused by the dopants. The ratios for the O species are shown in Fig. 2f, revealing a more homogeneous distribution of surface oxygen vacancies, surface-adsorbed molecular water, lattice oxygen, and surface-adsorbed hydroxyl groups or oxygen in the 1%-Bi-CeO₂ sample compared to other catalysts. The Bi 4f spectra (Fig. 2g) show two pairs of peaks: at 161.9 and 156.7 eV for metallic Bi 4f5/2 and Bi 4f7/2, and at 163.6 and 158.3 eV for Bi₂O₃. The XPS spectra and corresponding ratios (Fig. 2h) indicate that Bi₂O₃ is predominantly present on the surface of all Bi-containing CeO₂ catalysts, with the 1%-Bi-CeO₂ sample showing a surface composition of 7.4% metallic Bi and 92.6% Bi₂O₃. The XPS analysis confirms a strong electronic interaction between Bi and CeO₂, which optimizes the local electronic structure and enhances the intrinsic catalytic activity. The Ce, O and Bi species contents of x%-Bi-CeO₂ calculated from the fitted high-resolution XPS spectra are presented in Table S1.†

The microstructure and morphology of the prepared Bi-doped CeO₂ catalysts were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The progressively magnified SEM images (Fig. S2a–e†) of the CeO₂ sample display microspheres assembled by lamellars. The microspheres with a hollow structure (average diameter of 189.3 nm) and triangular nanoflakes can be confirmed from the TEM image (Fig. 3a, b and Fig. S2f, g†), which shows that the CeO₂ spheres were supported by the lamellar structure. The corresponding energy-dispersive X-ray spectroscopy (EDX) maps show that Ce and O elements are uniformly distributed in the nanoplates. The corresponding selected area electron diffraction (SAED) patterns of CeO₂ nanosheets and hollow nanospheres have three identical diffraction rings corresponding to the (1 1 1), (2 2 0), and (3 3 1) crystal planes of CeO₂. The high resolution TEM (HRTEM) image of the CeO₂ lamellar structure (Fig. 3c) showed obvious lattice fringes, measuring 0.31 nm and 0.27 nm, corresponding to the (1 1 1) and (2 0 0) crystal planes of CeO₂. The HRTEM image of fresh CeO₂ nanospheres shows an identical crystal plane with a ninety-degree angle between the (1 1 1) and (2 2 0) planes. 0.5%-Bi-CeO₂ exhibits the same micromorphology (Fig. S3 and S4†) as pure CeO₂ (Fig. S2†) but with an increased number of hollow-structured nanospheres and a reduced average sphere diameter of 129.7 nm. HRTEM showed obvious lattice fringes with adjacent interplanar spacings of 0.31, 0.27, and 0.19 nm, corresponding to the (1 1 1), (2 0 0), and (2 2 0) planes of CeO₂, consistent with the SAED patterns (Fig. S4e†). The EDX maps obtained from Fig. S4f† and presented in Fig. S4g–i† show the uniform distribution of Ce, Bi and O.

Fig. 3 TEM (a), enlarged magnification TEM image (b), EDX maps (selected area in (a)), SAED patterns, and HRTEM images (c) of pristine CeO₂; TEM (d), enlarged magnification TEM image (e), STEM image and corresponding EDX maps (f), HRTEM images (g), and the corresponding SAED patterns (inset) of 1%-Bi-CeO₂; (h) TEM image of 2%-Bi-CeO₂; (i) particle size distribution of CeO₂ and x%-Bi-CeO₂ (x = 0.5, 1, 2, 5); and HRTEM image (j), STEM image and corresponding EDX maps (k), and SAED patterns of 2%-Bi-CeO₂.

By continuously increasing the percentage of Bi to 1%, the uniform distribution of CeO₂ nanospheres can be confirmed (Fig. S5†). The TEM image (Fig. 3d) also shows the uniform distribution of hollow spheres on the lamellar structure, and the average diameter of the spheres was further decreased to 118.1 nm. Furthermore, the elemental maps obtained from the TEM image of Fig. 3f present a homogeneous dispersion of Ce, O, and Bi. Obvious lattice fringes can also be clarified from both the spheres and lamellar structure. As can be seen from the lamellar structure (Fig. 3g), adjacent lattice spacings of 0.31 nm and 0.33 nm were observed which belong to the (2 2 0) crystal plane of CeO₂ and the (0 1 2) crystal plane of Bi, respectively. The SAED image (Fig. 3g, inset) has four distinct diffraction rings corresponding to the (1 1 1), (2 2 0), (2 2 0), and (3 3 1) crystal planes of CeO₂. In addition, adjacent lattice fringes of 0.19 and 0.33 nm were observed from the hollow spheres, corresponding to the (2 2 0) crystal plane of CeO₂ and the (0 1 2) crystal plane of Bi, respectively. The SAED pattern (inset) displays four distinct diffraction rings, corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 3 1) crystal planes of CeO₂, respectively. The high dispersion of Bi dopants and the smallest hollow spheres would be conducive to optimizing the electronic properties and electrochemical interfaces, which are beneficial for enhancing the 2e-ORR performance. When the percentage of Bi increased to 2%, the morphology changed from a hollow nanosphere structure back to a thick layered structure of assembled large spheres and large particles (Fig. S6†), which may be attributed to the segregation phenomenon with the increase of the Bi content. Comparatively, the catalyst with a Bi percentage of 5% (Fig. S7†) showed the same morphology as 2%-Bi-CeO₂, but the sphere diameter increased further, and negligible Bi-CeO₂ spheres were observed. The segregation of Bi is unfavorable for ORR performance. The size distribution point plot in Fig. 3i indicates that 1%-Bi-CeO₂ has the smallest sphere diameter. Fig. 3h shows that the size of the lamellar structure increases sharply compared to pure CeO₂, 0.5%-Bi-CeO₂, and 1%-Bi-CeO₂ with an obvious crystal structure (Fig. 3j and Fig. S8†). Besides, the spheres changed from hollow to solid for 2%-Bi-CeO₂ and 5%-Bi-CeO₂, which corresponds to Bi particles shown in the EDX maps (Fig. 3k and Fig. S8g–i†). Furthermore, nitrogen adsorption–desorption tests were conducted on 1%-Bi-CeO₂ and CeO₂ (Fig. S9†), yielding specific surface areas of 75.5 m² g⁻¹ and 30.1 m² g⁻¹, respectively. These results show the significant increase in electrochemical interfaces achieved upon introducing bismuth.

Electrochemical ORR activities of the as-synthesized catalysts were measured by using a three-electrode configuration. Typical of the cyclic voltammogram (CV) curves of 1%-Bi-CeO₂, an obvious oxygen reduction peak at 0.60 V can be seen in the O₂-saturated 1 M KOH solution compared to the curve in the N₂-saturated solution (Fig. S10†), preliminarily confirming that 1%-Bi-CeO₂ exhibited ORR activity. Furthermore, the polarization curves are presented in Fig. 4a. Among the samples tested, 1%-Bi-CeO₂ exhibited the highest ring current compared to other x%-Bi-CeO₂, CeO₂, and the bare glassy carbon electrode (GCE), indicating the highest H₂O₂ production.

Fig. 4 (a) RRDE polarization curves of GCE, CeO₂ and x%-Bi-CeO₂ in 1 M KOH solution; (b) corresponding faradaic efficiency (%) and H₂O₂ selectivity (%) of CeO₂ and x%-Bi-CeO₂ in 1 M KOH solution; (c) mass activity of different electrocatalysts published recently and 1%-Bi-CeO₂ for H₂O₂ production, of which dashed lines represent data obtained in 0.1 M HClO₄ and the solid lines correspond to data obtained in 0.1 M KOH. (d) H₂O₂ yield rate versus potential under the O₂-saturated and air atmosphere; and (e) H₂O₂ yield rate versus cycle number under the O₂-saturated and air atmosphere.

To obtain the specific activities of the catalysts, the faradaic efficiency (FE%) and H₂O₂ selectivity (%) at various potentials were calculated using eqn (S1) and (S2),† and the results are plotted in Fig. 4b, which shows that 1%-Bi-CeO₂ achieves a high H₂O₂ selectivity of 62.3% at 0.40 V, respectively. The same conclusion can be drawn for ORR performance in the O₂-saturated 0.1 M KOH solution (Fig. S11a and b†), but the selectivity toward H₂O₂ was increased to 70% at 0.3 V. The electron transfer numbers (n) for the ORR were calculated using eqn (S3)† in various alkaline solutions, as shown in Fig. S12.† The results indicate that the ORR predominantly follows a two-electron transfer pathway. Notably, the reaction pathway shows an increased preference for the two-electron process as the alkalinity of the electrolyte solution decreases. To accurately determine the theoretical electron transfer number, polarization curves of the 1%-Bi-CeO₂ catalyst at different rotation speeds were plotted as shown in Fig. S13a,† which shows that the current density increased with increasing rotation speed. The corresponding K–L plots (Fig. S13b†) derived from the equations (eqn (S4) and (S5)†) showed an electron transfer number (n) of 1.92, 1.94, 1.99, 1.99, and 2.14 at potentials of 0.40 V, 0.45 V, 0.50 V, 0.55 V, and 0.60 V. Thus, it could be deduced that the 1%-Bi-CeO₂ catalyst follows a two-electron ORR pathway. The same conclusion can be drawn for ORR performance in the O₂-saturated 0.1 M KOH solution (Fig. S14†), and the selectivity toward H₂O₂ was increased to 70% at 0.30 V. The corresponding n values calculated from Fig. S13† to be 1.98, 1.98, 1.98, 1.98, and 2.11 at potentials of 0.40 V, 0.45 V, 0.50 V, 0.55 V, and 0.60 V, respectively, also indicated typical 2e-ORR performance. Additionally, the apparent diffusion coefficient in a 1 M KOH solution was estimated using the Levich equation (eqn (S5)†) at high overpotentials. The calculated value was approximately 5 × 10⁻⁵ cm² s⁻¹, indicating the excellent diffusion properties of 1%-Bi-CeO₂. The kinetic currents of 1%-Bi-CeO₂ in 1 M KOH solution, normalized by the mass of the loaded catalyst and corrected for iR drops under near-zero overpotential conditions, were compared to those of recently published state-of-the-art catalysts (Fig. 4c).^41–48 This analysis highlights the exceptional mass activity of 1%-Bi-CeO₂. Considering that the electrical double-layer capacitor (C_dl) value at the non-pseudocapacitance potential window is proportional to the electrochemically active surface area (ECSA), CV curves at a non-pseudocapacitance potential window of 0.80–1.0 V were plotted (Fig. S15†). After fitting, the C_dl value of 1%-Bi-CeO₂ was calculated to be 17.6 mF cm⁻² mg⁻¹ (Fig. S15d†). Moreover, the kinetic current obtained from the K–L analysis was normalized by the BET surface area (Fig. S9†), denoted as j^norm_K. The kinetic current density (j_K) at near-zero overpotential was determined using eqn (S5).† At near-zero overpotential, the j^norm_K calculated via eqn (S8)† is presented in Fig. S16.† As a result, 1%-Bi-CeO₂ exhibited a significantly higher current density compared to CeO₂, indicating superior reaction kinetics for 1%-Bi-CeO₂.

An H-cell configuration was employed at various potentials to investigate H₂O₂ production. The hydrogen peroxide production was calculated using eqn (S6).† The histogram of Fig. 4d shows the H₂O₂ yield under the O₂-saturated atmosphere, which is as high as 1.18 mol g_cat⁻¹ h⁻¹ at −0.1 V (vs. RHE), exceeding that of the previously reported catalysts (Table S2†). As expected, the optimized catalyst also shows good H₂O₂ production performance under natural air diffusion conditions. The production yield of H₂O₂ at 0.1 V (vs. RHE) reaches 0.92 mol g_cat⁻¹ h⁻¹. The H₂O₂ production yields remain stable from −0.10 V to 0.20 V, suggesting that the catalyst has a wide H₂O₂ catalytic potential window. In addition, 1%-Bi-CeO₂ still maintains an expected H₂O₂ production rate of 1.15 mol g_cat⁻¹ h⁻¹ after recycling 4 times under O₂-saturated conditions, which implies that the catalyst has good reusability (Fig. 4e). Furthermore, the H₂O₂ production of the catalyst still remains at 0.88 mol g_cat⁻¹ h⁻¹ under air-saturated conditions. The faradaic efficiency calculated based on eqn (S7) is shown in Fig. S14a,† which shows that 1%-Bi-CeO₂ exhibits a high faradaic efficiency of 56.0% under oxygen-saturated conditions (0.10 V vs. RHE) (Fig. S17a†) and remains stable after four cycles (Fig. S17b†).

Theoretical calculations were conducted via the DFT based first-principles calculations. The Gibbs free energy change (ΔG) for each elementary step in the ORR process is calculated using eqn (S9).† As shown in Fig. 5a, the optimized model of Bi-CeO₂ was established based on the HRTEM and SAED analyses (Fig. 3) with different stabilized adsorption configurations of the ORR intermediate (OOH*). During the process, O₂ binds to the catalyst surface and further transfers an electron to form OOH*, followed by another electron transfer to form H₂O₂. To analyze the contribution of the electronic structure to the catalytic efficiency, density of states (DOS) plots were derived (Fig. 5b). The overall electronic structures vary after the introduction of Bi species. As for 1%-Bi-CeO₂, we notice that the Ce (1 1 1) crystal surface shows the highest electron density near the FE. This suggests that the electron transfer capacity is subtly affected by the Bi dopant. To further reveal the electrocatalytic process, the adsorption energies of 1%-Bi-CeO₂ and CeO₂ in the ORR process are plotted in Fig. 5c. Since the whole process is dominated by the following reaction step: O₂(g) → OOH*, 1%-Bi-CeO₂ shows a higher reaction tendency with a corresponding ΔG value of −0.93 eV, much higher than that of Bi-CeO₂ (−0.55 eV), which is consistent with the electrochemical results, providing theoretical proof for excellent 2e-ORR via simple Bi-doping in CeO₂. Accordingly, the Bi dopants help CeO₂ to lower the reaction barrier from O₂(g) to OOH* (Fig. 5d), which is beneficial for H₂O₂ production.

Fig. 5 (a) Schematic illustration of the ORR process of x%-Bi-CeO₂; (b) DOS image of CeO₂ and 1%-Bi-CeO₂; (c) reaction free energy change at different ORR steps of pristine CeO₂ and 1%-Bi-CeO₂; and (d) schematic diagram of the structure and reaction principle of the 1%-Bi-CeO₂ catalyst.

Considering the efficiency of H₂O₂ in degrading organic dyes, a 1%-Bi-CeO₂ catalyst was employed for in situ generating H₂O₂ for the degradation of RhB. The standard curve for RhB degradation is shown in Fig. S18.† The dual cathode configuration is applied as shown in Fig. 6a. After four consecutive degradation cycles, the RhB removal efficiency is maintained at 82.6% (Fig. 6b). The superior degradation efficiency is due to the excellent recyclability of the 1%-Bi-CeO₂ cathode and stainless-steel mesh (SSM), where the SSM has the advantages of low cost, high chemical stability, and high catalytic activity for converting H₂O₂ to ˙OH.⁴⁹ EPR spectroscopy with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the trapping agent showed a gradual increase in the signals of the DMPO-˙OH adducts (Fig. 6c) after 0, 5, and 10 min of SSM cathode operation, confirming the existence of ˙OH. Accordingly, this novel dual cathode was employed to further investigate its efficiency in RhB degradation. The removal efficiency decreases rapidly with the addition of TBA (Fig. 6d). This rapid decrease in removal efficiency after the addition of TBA confirms that ˙OH is the primary oxidant in the system, which indirectly proves that the SSM cathode plays an important role in transforming the in situ generated H₂O₂ into ˙OH.

Fig. 6 (a) Schematic illustration of the dual-cathode configuration; (b) RhB degradation comparison rate versus cycle number using the dual-cathode system; (c) EPR spin trapping with dimethyl pyridine N-oxide (DMPO) at 0 min, 5 min, and 10 min operation of the 1%-Bi-CeO₂ and SSM cathodes; and (d) RhB degradation efficiency with/without tert-butanol (TBA) addition.

Conclusions

In summary, the Bi-CeO₂ catalyst with a hollow sphere/lamellar composite morphology and highly dispersed Bi dopants was successfully constructed for the electrochemical production of H₂O₂ by the 2e-ORR. Bi dopants played a significant role in optimizing the electronic structure and morphology. As a 2e-ORR catalyst, 1%-Bi-CeO₂ demonstrated the highest H₂O₂ output rate and good cyclability, much higher than that of pristine CeO₂ and other x%-Bi-CeO₂. Additionally, the device assembled with the optimized 1%-Bi-CeO₂ catalyst demonstrated significantly faster RhB degradation efficiency. DFT calculations revealed that the rate determining step of the ORR was optimized by introducing Bi dopants. The excellent catalytic performance can be mainly ascribed to the highly dispersed Bi dopants and unique hollow sphere/lamellar composite morphology, which respectively enhanced the intrinsic activity by optimizing the electronic structure and increased the ECSA by providing large electrochemical interfaces. This work will open up an exciting new avenue for the design of high-dispersive heteroatom-doped catalysts for the 2e-ORR in in situ wastewater treatment.

Author contributions

Q. Y.: performed the synthesis and physical and electrochemical characterization studies. Q. Y., J. W. and L. G.: conceived and designed the experiments. Q. Y. and C. S.: wrote the manuscript with assistance from J. W., L. G., C. M., and L.S. H. L., C. S., and L. S.: helped to perform the experiments. All authors participated in the analysis of the data. All authors have given approval to the final version of the manuscript.

Data availability

Data for this article, including original data, are available at the Science Data Bank at https://www.scidb.cn/en/s/NBNjeq.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 22202114), the Natural Science Foundation of Shandong Province (No. ZR2022MB124 and ZR2022QB028), the China Postdoctoral Science Foundation (No. 2021M701694), and the Postdoctoral Science Foundation of Jiangsu Province (No. 1006-YBA21038). The Central Laboratory of Qingdao Agriculture University and Qingdao Engineering Research Center of Agricultural Recycling Economy Materials are also acknowledged for help with physical characterization.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00075k

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