Electrochemical reconstruction of CuMoO₄ into CuO nanosheets for efficient electro-oxidation of 5-hydroxymethylfurfural

Abstract

The development of efficient electrocatalysts for the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is crucial for sustainable biomass valorization. Herein, CuMoO₄ was synthesized by using a novel method via anodic oxidation of copper foam in Na₂MoO₄ solution. Then the CuMoO₄ precursor was electrochemically reconstructed into a CuO catalyst (ER-CuO) via molybdate ion leaching. The ER-CuO exhibits a nanosheet morphology with abundant active sites, facilitating rapid charge transfer and enhanced electrocatalytic activity for the HMF oxidation reaction (HMFOR). ER-CuO requires only 1.43 V to achieve 50 mA cm⁻², significantly lower than that of CuO. At a potential of 1.55V, ER-CuO achieves a high FDCA yield of 97.5% with a faradaic efficiency (FE) of 98.3%, along with remarkable stability over 10 cycles. The electrochemical analysis demonstrated that the HMFOR process occurs via an indirect oxidation mechanism mediated by the Cu²⁺/Cu³⁺ redox couple. Moreover, the ER-CuO exhibits abundant oxygen vacancies, which facilitate the generation of Cu³⁺ active species, thus contributing to improved HMFOR activity. This work provides a rational strategy for designing high-performance electrocatalysts for the HMFOR through electrochemical reconstruction.

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Introduction

The rapid development of the economy has accelerated the consumption of fossil energy.¹ As a renewable carbon-based material, biomass has become a green alternative for fossil energy due to its abundance and renewability.² 5-Hydroxymethylfurfural (HMF), as the top biomass platform molecule, can be oxidized to 2,5-furandicarboxylic acid (FDCA), which has a conjugated carbon ring and diacid structure similar to terephthalic acid (TPA), and can replace TPA to synthesize biobased polyesters with better performance, reducing dependence on fossil resources.³ Compared to the conventional aerobic HMF oxidation reaction (HMFOR), the electrocatalytic approach offers distinct advantages including milder reaction conditions (ambient temperature and pressure) and high efficiency, and has therefore attracted considerable research attention in recent years.

Transition metal-based electrocatalysts have shown great promise for the HMFOR owing to their superior catalytic performance and cost-effectiveness. In particular, nickel (Ni)- and cobalt (Co)-based materials have been widely employed as active components. However, their strong oxygen evolution reaction (OER) activity presents a major challenge, as the OER competes severely with the HMFOR under high current densities, reducing faradaic efficiency.^4–6 In contrast, Cu-based catalysts exhibit relatively low OER activity, making them promising alternatives to Ni and Co counterparts.⁷ For instance, Shi et al. reported that Cu₂O demonstrates a higher activity for the formate oxidation reaction (FOR) but significantly lower OER activity compared to NiO.⁸ Similarly, Wang et al. found that CuO requires a higher overpotential for the OER than CoOOH and NiOOH, yet it exhibits superior catalytic performance in the glucose oxidation reaction (GOR).⁷ These findings underscore the potential of Cu-based catalysts in selective biomass electro-oxidation.

Transition metal molybdates have been demonstrated as effective catalysts in the field of electro-oxidation reactions, such as the OER,^9–11 HMFOR,^12,13 methanol electro-oxidation (MOR),^14,15 and glycerol oxidation reaction (GOR).¹⁶ It is noteworthy that the anodic reconstruction of transition metal molybdates into the corresponding oxides or hydroxides has become an important strategy for developing efficient biomass oxidation catalysts. For example, Wang et al. reconstructed nickel molybdate (NiMoO₄) using cyclic voltammetry (CV) and successfully prepared a molybdenum-doped hydroxide nickel oxide (Mo–NiOOH) catalyst, which exhibits a high FE of 84.7% toward glycerol oxidation to formate.¹⁶ Wang et al. prepared a novel Fe₂O₃-modulated P-doped CoMoO₄ catalyst on nickel foam. The electrochemical reconstruction of CoMoO₄ generates the active hydroxylated cobalt oxide (CoOOH) phase with abundant oxygen vacancies.¹⁷ Similarly, Shen et al. demonstrated self-reconstruction of Se-doped CoMoO₄ produce γ-CoOOH, which exhibits a larger specific surface area and more active sites than the directly synthesized counterpart, thus exhibiting superior OER performance.¹⁸ These research results verified the feasibility of electrochemical reconstruction of transition metal molybdate for the preparation of efficient catalysts. However, reconstruction of copper molybdates into active copper-based materials has never been reported for the HMFOR.

Herein, we prepared an electrochemically reconstructed CuO (ER-CuO) catalyst via anodic oxidation of the CuMoO₄ precursor. As a novel HMFOR catalyst, the ER-CuO only requires 1.43 V to derive 50 mA cm⁻², which is 130 mV lower than that of the CuO counterpart. Moreover, ER-CuO also achieves a high FDCA yield of 97.5% and FE of 98.3% at 1.55 V, as well as good electrochemical stability. X-ray photoelectron spectroscopy (XPS) analysis confirmed the existence of oxygen vacancies in ER-CuO, which facilitated the generation of more active sites. Multi-potential step measurements further demonstrated that ER-CuO exhibited a greater number of active sites participating in the HMFOR. Moreover, in situ electrochemical impedance spectroscopy (EIS) revealed that ER-CuO enhanced the electron transfer rate, thereby significantly improving HMFOR activity. This study provides a potential modification strategy for the development of high-performance HMFOR electrocatalysts through electrochemical reconstruction.

Experimental section

Preparation of CuMoO₄

CuMoO₄ was prepared on Cu foam (CF) using an anodic galvanostatic oxidation method. First, a piece of CF (1 × 1 cm²) was pretreated with ultrasonication to remove the surface oxides in ethanol, 3 M HCl and DI water for 10 min, respectively. Then, the pre-treated copper foam was immersed in a 1.0 M Na₂MoO₄ aqueous solution and subjected to electro-oxidation for 540 seconds under a constant current density of 30 mA cm⁻², employing a standard three-electrode system with an Ag/AgCl reference electrode. Subsequently, the sample was thoroughly rinsed with deionized water to remove residual electrolytes, yielding the CuMoO₄ precursor. CuWO₄ and Cu₃(VO₄)₂ were synthesized using an identical procedure, with the only difference being that the metal salts used in the electro-oxidation step were replaced with 1.0 M Na₂WO₄ and 1.0 M Na₃VO₄.

Preparation of ER-CuO

The ER-CuO catalyst was prepared by electro-oxidation of CuMoO₄ in an alkaline electrolyte. Specifically, a piece of the CuMoO₄ precursor (1 × 1 cm²) was used as the working electrode, and a 1.0 M KOH aqueous solution of KOH was used as the electrolyte. Electrochemical reconstruction was then performed by anodizing CuMoO₄ at 1.5 V vs. reversible hydrogen electrode (RHE) until no obvious current was observed.

Electrochemical measurements

All electrochemical measurements were performed on a CHI760E electrochemical workstation using a three-electrode system. 1.0 M KOH electrolyte without and with 50 mM HMF was used to evaluate OER and HMFOR performance, respectively. Linear sweep voltammetry (LSV) was conducted at potentials ranging from 1.0 to 1.8 V vs. RHE at a scan rate of 10 mV s⁻¹. CV was calculated at potentials ranging from −1.0 to 1.0 V vs. Hg/HgO. The electrochemical surface area (ECSA) was calculated using the formula ECSA = C_dl/C_s. Here, C_dl is the electrochemical double-layer capacitance obtained via CV curves collected at different scan rates in the non-faradaic region (0.45–0.55 V vs. Hg/HgO). The value of C_s is assumed to be 0.04 mF cm⁻².

Product quantification

All the organics were determined by the CLC-3200 high performance liquid chromatograph with a UV-vis detector (wavelength of 265 nm). HPLC was fitted with an Xtimate Sugar-H column using a mobile phase (5 mM aqueous H₂SO₄) at a flow rate of 0.6 mL min⁻¹ at a temperature of 65 °C. The conversion of HMF (%), the yield (%) and FE (%) of FDCA were calculated by using the following equations:Here, F is the Faraday constant (96 485 C mol⁻¹).

Results and discussion

The synthesis of the ER-CuO catalyst is illustrated in Scheme 1. First, the CuMoO₄ precursor was synthesized by anodic oxidation of CF in a Na₂MoO₄ solution. The CF reacted with MoO₄²⁻ in the solution to generate CuMoO₄ on its surface. The structural instability of CuMoO₄ is directly evidenced by the irreversible change in LSV curves and the significant leaching of Mo species as detected by XPS (Fig. S1 and S2). Therefore, the CuMoO₄ precursor was reconstructed at a constant potential of 1.5 V in a KOH solution, where the leaching of MoO₄²⁻ induced the transformation of the CuMoO₄ into ER-CuO catalyst. Fig. S3 demonstrates that the ER-CuO catalyst, derived from the reconstruction of CuMoO₄, exhibits superior electrocatalytic performance compared to those derived from CuWO₄ and Cu₃(VO₄)₂ pre-catalysts. The morphologies of CuMoO₄ and ER-CuO catalysts are characterized by scanning electron microscopy (SEM). As shown in Fig. 1a, CuMoO₄ presents a nanoparticle morphology. The transmission electron microscopy (TEM) image (Fig. 1b) further reveals that CuMoO₄ nanoparticles are composed of numerous nanosheets. The high-resolution transmission electron microscopy (HRTEM) image clearly shows the lattice fringes with an interplanar distance of 0.303 nm and 0.374 nm (Fig. 1c), which can be assigned to the (−212) and (201) lattice plane of CuMoO₄, respectively. Energy dispersive spectroscopy (EDS) analysis confirms that Cu, Mo, and O were evenly distributed throughout the sample (Fig. S4). Notably, anodic oxidation of CuMoO₄ induces a remarkable morphological transformation, yielding ER-CuO catalysts with distinctly different nanosheet architectures compared to the precursor (Fig. 1d and e), which clearly demonstrates the substantial structural evolution occurring during the electrochemical oxidation process. Moreover, the lattice fringes with lattice spacing of 0.254 nm were observed in the HRTEM image, which can be attributed to the (11−1) plane of CuO (Fig. 1f). EDS analysis further reveals that in comparison with pristine CuMoO₄, the ER-CuO possesses a substantially higher oxygen content of 48.96% while exhibiting a markedly reduced Mo content of only 0.23% (Fig. 1g–j and Fig. S5, Table S1). This observation indicates the near-complete leaching of molybdate species during the anodic oxidation of CuMoO₄.

Scheme 1 Schematic diagram of the preparation process of ER-CuO.

Fig. 1 (a) SEM image, (b) TEM image, and (c) HRTEM image of CuMoO₄. (d) SEM image, (e) TEM image, (f) HRTEM image, and (g)–(j) EDS mappings of ER-CuO.

The crystal structures of CuMoO₄ and ER-CuO are characterized by X-ray diffraction (XRD). As seen in the XRD pattern in Fig. S6, the CuMoO₄ catalyst exhibits only three characteristic peaks of copper from the CF at 43.3°, 50.4°, and 74.1°, while no other detectable diffraction peaks are observed. This can be attributed to the poor crystallinity and small size of CuMoO₄ nanocrystals. In contrast, the ER-CuO displays well-defined diffraction peaks at 35.5°, 38.7°, and 48.7°, which are attributed to the (11−1), (111), and (20−2) planes of CuO (PDF#48-1548), respectively (Fig. 2a), providing another evidence for the reconstruction of CuMoO₄ into ER-CuO. Raman spectroscopy was employed to further characterize the structure of the catalysts. As shown in Fig. 2b, three weak peaks of CuMoO₄ appear at 324, 612, and 820 cm⁻¹, which are attributed to the lattice vibrations of the Mo–O bond. Another peak at 928 cm⁻¹ is assigned to the stretching vibration of the Mo=O bond.^19,20 Meanwhile, the peaks of ER-CuO at 298 and 628 cm⁻¹ are attributed to CuO, consistent with XRD and HRTEM results.^21–23 Based on this evidence, we concluded that the CuMoO₄ precursor was converted into ER-CuO during the anodic oxidation process. Furthermore, XPS was conducted to investigate the valence states. As shown in the survey spectrum (Fig. 2c), CuMoO₄ exhibits the signal of Cu, O, and Mo elements, while the reconstructed ER-CuO only contains Cu and O elements. Moreover, the high-resolution Mo 3d XPS spectrum (Fig. 2d) of CuMoO₄ displays distinguished peaks located at 232.1 and 235.2 eV, which can be attributed to the characteristic peak of Mo⁶⁺ in CuMoO₄.^24,25 In sharp contrast, no detectable Mo signal was observed for ER-CuO, indicating that the MoO₄²⁻ species was etched away, consistent with the EDS results. In the Cu 2p region (Fig. 2e), two peaks at 932.4 and 952.4 eV belong to Cu⁺ species, and the peaks at 934.7 and 954.7 eV are attributed to Cu²⁺ species. In comparison with CuMoO₄, the ER-CuO exhibits a higher content of Cu⁺, which may be rationalized by the presence of abundant oxygen vacancies.^26–29 The O 1s XPS spectra (Fig. 2f) show peaks at 532.6, 530.9, and 529.2 eV, which belong to surface-adsorbed water (O_Ads), oxygen vacancy (O_V), and lattice oxygen (O_Latt), respectively.³⁰ Note that the oxygen vacancy signal for ER-CuO is significantly higher than that for CuO prepared by using the hydrothermal method and calcination (see the SI for details, Fig. S7 and S8), confirming the presence of abundant oxygen vacancies in the ER-CuO.³¹

Fig. 2 (a) XRD patterns of ER-CuO. (b) Raman spectrum of CuMoO₄ and ER-CuO. XPS spectra of (c) survey, (d) Mo 3d, and (e) Cu 2p for CuMoO₄ and ER-CuO. (f) XPS spectra of O 1s for CuO and ER-CuO.

The electrocatalytic performances of ER-CuO and CuO catalysts were evaluated using a three-electrode system in 1.0 M KOH with and without 50 mM HMF.³² As shown in Fig. 3a, the LSV curve of ER-CuO for the OER shows that a potential of 1.64 V was required to reach a current density of 50 mA cm⁻² and a potential of 1.75 V was required to attain a current density of 200 mA cm⁻². However, when 50 mM HMF was introduced, the potentials needed to reach the same current density reduced to 1.43 V and 1.51 V, which were 210 mV and 240 mV lower than those of the OER, suggesting that the HMFOR is more favorable than the OER. Subsequently, the HMFOR performance of ER-CuO and CuO was evaluated in 1.0 M KOH with 50 mM HMF. CuO required potentials of 1.56 V and 1.64 V to achieve current densities of 50 mA cm⁻² and 100 mA cm⁻², respectively. In contrast, ER-CuO exhibited significantly lower potentials, requiring only 1.43 V and 1.47 V to reach the same current densities, corresponding to reductions of 130 mV and 170 mV, demonstrating that ER-CuO possesses superior HMFOR activity compared to CuO (Fig. 3b). In addition, the electrocatalytic performance of ER-CuO was compared against that of previously reported catalysts in 1.0 M KOH electrolyte containing 10 mM or 50 mM HMF. As summarized in Fig. S9 and Tables S2, S3, ER-CuO required a lower applied potential to achieve the same current density, underscoring its promise as an efficient Cu-based electrocatalysts for HMF oxidation. Moreover, the Tafel slopes of ER-CuO was calculated to be 109 mV dec⁻¹ for the HMFOR (Fig. 3c). The smaller Tafel slope of the ER-CuO electrode than that of CuO (242 mV dec⁻¹) and CF (111 mV dec⁻¹) suggests faster catalytic kinetics over ER-CuO.^33,34 EIS of ER-CuO and CuO was carried out to further estimate reaction kinetics. The Nyquist plot of the ER-CuO (Fig. 3d) exhibits the smallest semicircle radius in the high frequency region and a much smaller R_ct value of 1.913 Ω as compared with CuO (9.889 Ω), signifying a faster electron transfer process of ER-CuO.^35,36 Besides, to study the inherent activity, ECSA of the catalysts for the HMFOR was compared by measuring C_dl (Fig. S10).^37,38 The C_dl of ER-CuO was calculated as 470 mF cm⁻², higher than that of CuO (404 mF cm⁻²), demonstrating that ER-CuO provides a larger active area for the HMFOR (Fig. 3e).^39,40 Moreover, after normalizing the LSV curves by the ECSA (Fig. 3f), ER-CuO still shows a higher current density than CuO, demonstrating that the catalyst derived from electrochemical reconstruction possesses higher inherent activity. Meanwhile, the origin of the enhanced HMFOR activity over ER-CuO was further probed by calculating the turnover frequency (TOF).⁴¹ As shown in Fig. S11, the TOF value of ER-CuO during the HMFOR process was markedly higher than that of CuO, suggesting a superior intrinsic activity. To rule out the possibility that the residual Mo species contributed to the catalytic activity, HMFOR activity of CuO was assessed in electrolytes with varying concentrations of Na₂MoO₄. Fig. S12 shows that regardless of the concentration of Na₂MoO₄ added, the performance remained largely unchanged. This proves that the HMFOR activity of CuO was unaltered by the presence of MoO₄²⁻.

Fig. 3 (a) LSV curves in 1.0 M KOH with and without 50 mM HMF of ER-CuO. (b) LSV curves and (c) Tafel plots of ER-CuO, CuO, and Cu foam in 1.0 M KOH with 50 mM HMF. (d) Nyquist plots, (e) calculated double layer capacitance, and (f) ECSA normalized LSV curve of ER-CuO and CuO.

To quantify reaction products of the HMFOR, potentiostatic electrolysis was carried out at 1.55 V, and the composition of electrolyte was analyzed by high performance liquid chromatography (HPLC, Fig. S13). It is known that HMF-to-FDCA electrocatalysis may proceed through two different pathways, as displayed in Fig. 4a.^42–44 One method involves oxidation of the aldehyde groups in HMF to carboxyl groups first, thereby generating 5-hydroxymethyl-2-furan carboxylic acid (HMFCA); the other method involves oxidation of the alcohol hydroxyl groups in HMF to aldehyde groups, thereby forming 2,5-diacetyl furan (DFF). Both of these intermediates are subsequently oxidized to 2-formyl-5-furancarboxylic acid (FFCA), and eventually to FDCA.⁴⁵ The results obtained through different charges via HPLC are given in Fig. 4b to show the reaction process of the catalytic HMFOR. The signal attributed to HMF at the retention time of 7.7 min continuously decreased in intensity while the signal of FDCA at 4.3 min gradually increased with the progress of the reaction. HMFCA (5.4 min) is detected as the major intermediate product, suggests that the HMFOR over ER-CuO follows the HMFCA path as shown in Fig. 4a. Moreover, complete HMF conversion was achieved within 48 min (Fig. 4c). The effect of applied potential on HMFOR was also investigated. As presented in Fig. 4d, both the yield and FE of FDCA increased over the potential range of 1.4 to 1.55 V. Notably, at 1.55 V, an FDCA yield of 97.5% with a FE of 98.3% was achieved. Furthermore, the ER-CuO electrode demonstrated remarkable durability, maintaining FDCA yields and FE above 90% over 10 consecutive cycles (Fig. 4e and Fig. S14). In contrast, the CuO catalyst delivered an initial FDCA yield of 64.9% and FE of 57.3%, which remained largely stable throughout the stability test (Fig. S15 and S16). The structural stability and electrocatalytic performance of the ER-CuO catalyst were evaluated. As shown in Fig. S17, XRD analysis after 10 HMF oxidation cycles confirmed that its crystal structure remained intact, indicating robust stability. Furthermore, compared with state-of-the-art Cu-based catalysts for the HMFOR, ER-CuO achieved the highest FE (Fig. S18 and Table S4). These results collectively demonstrate that ER-CuO possesses both exceptional structural integrity and superior catalytic activity.

Fig. 4 (a) Two oxidation pathways of HMF. (b) HPLC chromatogram. (c) Conversion of HMF and yield of FDCA and other products during HMFOR on ER-CuO. (d) Yield and FE toward FDCA under different applied potentials. (e) Stability test of ER-CuO for 10 cycles.

The effect of different KOH concentrations and different HMF concentrations on the HMFOR were also investigated. As shown in Fig. S19a, the HMFOR performance was the poorest in 0.1 M KOH. In contrast, the catalytic activities in 1.0 M and 2.0 M KOH were comparable. Within the 1.0 M KOH electrolyte, the lowest activity was observed with the addition of 10 mM HMF, whereas 50 mM and 100 mM HMF concentrations resulted in similar performance (Fig. S19b). The product analysis is presented in Fig. S20, and the FE of converting HMF to FDCA is the highest in 1.0 M KOH solution.

To investigate the reaction mechanism, a multi-potential step test was carried out.^46–48 As shown in Fig. 5a, from 0 to 100 s, high-valence Cu³⁺ species were enriched through electro-oxidation of ER-CuO at 1.5 V. Then the potential was converted into the open circuit potential (OCP). After standing in OCP for 100 s, the potential was then switched to 0.8 V to reduce the electro-generated Cu³⁺ species back to Cu²⁺. Notably, no cathodic current was observed at 0.8 V in 1.0 M KOH with 50 mM HMF, indicating that the accumulated Cu³⁺ was consumed by HMF via a spontaneous non-faradaic redox reaction.^49–51 This result suggests that the HMFOR over ER-CuO proceeds through a so-called “indirect mechanism”, as illustrated in Fig. 5b.^52–55 First, during the reaction process, the CuO component in the ER-CuO catalyst is oxidized to CuOOH. Then, the generated CuOOH species serves as the catalytic site for the HMFOR, oxidizing HMF to FDCA, and is reduced back to CuO, achieving an indirect reaction involving Cu²⁺/Cu³⁺. To reveal the advantage of ER-CuO for the “indirect” HMFOR, CV measurements were conducted in 1.0 M KOH. As exhibited in Fig. 5c, an obvious oxidation peak was observed around 1.0 V, which could be attributed to Cu⁰/Cu⁺ → Cu²⁺ oxidation. Another small oxidation peak at around 1.4 V originates from Cu²⁺ → Cu³⁺ oxidation. In comparison with CuO, the ER-CuO shows a larger oxidation peak at 1.4 V, suggesting the presence of more Cu³⁺ sites.^56–58

Fig. 5 (a) Multi-potential step curves of ER-CuO in 1.0 M KOH without and with 50 mM HMF. (b) Schematic illustration of HMF indirect oxidation mechanism. (c) CV curves of ER-CuO and CuO in 1.0 M KOH. (d) Density of Cu²⁺/Cu³⁺ redox site for ER-CuO and CuO. Bode phase plots of ER-CuO in 1.0 M KOH (e) without HMF and (f) with 50 mM HMF.

Moreover, multi-potential measurements were performed to quantify the number of Cu³⁺ sites on different catalysts. As shown in Fig. 5d and Fig. S21, the number of Cu³⁺ sites for ER-CuO was determined to be 0.3 nmol cm_ECSA⁻², much higher than that for CuO (0.18 nmol cm_ECSA⁻²), demonstrating that ER-CuO provides more Cu³⁺ active sites for the HMFOR. The reaction kinetics of OER and HMFOR were further investigated by using in situ EIS (Fig. 5e, f and Fig. S22).^59–62 The corresponding Bode plots are presented in Fig. 5. Under 1 M KOH conditions (Fig. 5e and Fig. S22a), ER-CuO and CuO exhibited OER in the low-frequency region at 1.6 and 1.7 V, respectively, in agreement with the OER LSV results.⁶³ Upon the introduction of 50 mM HMF (Fig. 5f and Fig. S22b), ER-CuO demonstrated a markedly reduced phase angle compared to CuO, suggesting a faster electron transfer. This finding is further corroborated by the Nyquist plots of HMFOR and OER (Fig. S23). Additionally, the EIS data were fitted using an optimized equivalent circuit model (Tables S5–S8). Notably, the charge transfer resistance (R_ct) of ER-CuO remained substantially lower than that of CuO across all applied potentials, further confirming the enhanced oxidation kinetics of the HMFOR on ER-CuO.^64–67 Overall, ER-CuO possesses abundant oxygen vacancies, which facilitate the oxidation of Cu²⁺ to Cu³⁺, thereby enhancing the availability of active sites for the HMFOR. Moreover, ER-CuO exhibits lower charge transfer resistance, further accelerating reaction kinetics. As a result, ER-CuO demonstrates excellent catalytic activity for the HMFOR.

Conclusions

In summary, we successfully synthesized an ER-CuO catalyst via electrochemical reconstruction of CuMoO₄. The ER-CuO catalyst demonstrates exceptional performance in the HMFOR, achieving a current density of 100 mA cm⁻² at a potential of 1.47 V, significantly outperforming CuO. Furthermore, at 1.55 V, ER-CuO exhibits a high FDCA yield of 97.5% with a FE of 98.3%, alongside remarkable electrochemical stability. The XPS and multi-potential step measurement studies confirmed the formation of oxygen vacancies in ER-CuO, which is favorable for the oxidation of Cu²⁺ to generate more Cu³⁺ active sites. This work not only provides a novel method to synthesize CoMoO₄, but also reveals the structure–activity relationship of Cu-based catalysts.

Author contributions

The manuscript was completed through contributions of all authors. Specifically, the division is listed as follows: conceptualization, K. Y., Y. W., and R. G.; funding acquisition and resources, R. G.; investigation and methodology, K. Y.; validation, K. Y., X. Y., and R. G.; supervision, J. W., R. G.; writing–original draft, K. Y.; and writing–review and editing, K. Y. and R. G. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. Supporting data including experimental methods, supporting figures, and tables referenced in the main text have been included as part of the SI. See DOI: https://doi.org/10.1039/d5nj03680a.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 22402107) and the Natural Science Foundation of Shandong Province (ZR2023QB094).

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