Cu³⁺/Ni³⁺ dual active sites for high-voltage driven electrocatalytic production of 2,5-furanedicarboxylic acid

Abstract

High voltage is key to achieving large-scale production of 2,5-furanedicarboxylic acid (FDCA) and highlights the future application potential of electrocatalysis technology in biomass chemical engineering. However, high voltage in the electrocatalytic system significantly promotes the oxygen evolution reaction (OER), thereby reducing the selectivity and Faraday efficiency (FE) of FDCA. In this work, Cu–Ni₃S₂ was successfully fabricated for efficient oxidation of HMF to FDCA, achieving desirable Faraday efficiency (FE) at high voltage. The synergistic action of Cu³⁺ and Ni³⁺ species effectively inhibited the OER and further achieved 100% selectivity and FE for FDCA production at a high voltage of 1.55V_RHE, which exceeds the operating voltage of the reported Ni₃S₂ system. Therefore, this work provides a new insight into the application of Ni₃S₂ catalysts at high voltage, advancing the transformation of biomass-derived platform compounds into high-value chemicals.

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1. Introduction

As the world's reliance on fossil fuels intensifies, the development of renewable alternatives has become imperative.^1–5 Biomass is emerging as a carbon-neutral resource, with 5-hydroxymethylfurfural (HMF) standing out as a key platform compound. Consequently, the U.S. Department of Energy has designated HMF as one of twelve top-priority platform chemicals to accelerate the replacement of petrochemical products.^6–8 HMF can be oxidized to 2,5-furandicarboxylic acid (FDCA), which is a green alternative to replace the current petroleum-based monomer for polyethylene terephthalate (PET) production.⁹ Compared to the limitations of the traditional HMF oxidation reaction (HMFOR),¹⁰ the electrocatalytic HMFOR route could operate under mild reaction conditions, significantly reduce the energy consumption, and achieve higher product selectivity and yield.^11,12 Recently, at 1.5V_RHE and below, FDCA yields have achieved relatively satisfactory results, reaching a maximum of 100%.^13–17 However, the rapid large-scale production of FDCA imposes a critical requirement for high voltage adaptation. Meanwhile, maintaining excellent faradaic efficiency (FE) and reaction kinetics at high voltage is important for the industrial application of the HMFOR. Therefore, it is urgent to design catalysts, which can withstand high voltages and establish a highly efficient HMFOR system.

Nickel sulfide (Ni₃S₂) demonstrates great potential in the HMFOR due to its high theoretical activity, multi-electron oxidation capability, and cost-effectiveness.^18–22 For example, Yi et al. proposed that NiS_X/CB could achieve 100% HMF conversion, 98% FDCA yield, and 100% FE at 1.45V_RHE.²³ Chen et al. reported that Ni₃S₂/NF realized 98.8% FDCA yields and 97.6% FE by modulating Ni–S bonds in Ni₃S₂, with complete HMF conversion at 1.45V_RHE.²⁴ The traditional Ni₃S₂ materials achieve 100% FE below 1.5V_RHE, but the application of Ni₃S₂ at voltages above 1.5V_RHE is restricted, due to the competition between the oxygen evolution reaction (OER) and the HMFOR.²⁵ Enhancing the FE of Ni₃S₂ at voltages above 1.5V_RHE requires suppressing the OER to achieve 100% FE. Furthermore, Cu could hinder OH⁻ deprotonation to O⁻, consequently inhibiting the OER in the HMFOR system. For example, Chen et al. revealed that the Ni–Cu/NF catalyst has achieved nearly 100% FE and yield at 1.50V_RHE.¹⁷ Wang et al. reported that Co₉S₈–Ni₃S₂/Cu effectively suppressed the OER and accelerated HMFOR kinetics.²⁶ Therefore, construction of Cu-modified Ni₃S₂ (Cu–Ni₃S₂) will be a feasible strategy to achieve high FE at high voltage, but the corresponding process mechanism has not yet been reported.

In this work, a novel Cu–Ni₃S₂ catalyst has been successfully fabricated through hydrothermal and electroplated methods. CuOOH and NiOOH active species could be further obtained after electrochemical activation. The introduction of Cu not only addressed the FE limitations of Ni₃S₂ at high voltages but also improved the reaction activity. Experimental results demonstrated that the Cu–Ni₃S₂ catalyst achieved nearly 100% FDCA yield and 100% FE at 1.55V_RHE, which could also maintain high FE after five consecutive cycling experiments. Therefore, this work provides a promising approach to address the FE issue of Ni₃S₂ at high voltage, which can be applied to establish electrocatalytic systems for industrial FDCA production.

2. Results and discussion

The preparation flowcharts of Cu–Ni₃S₂ and Ni₃S₂ are illustrated in Fig. 1a. Ni₃S₂ is synthesized through a hydrothermal method, and then Cu is electro-deposited on the surface of Ni₃S₂. The scanning electron microscopy (SEM) images show that Cu–NF has a sheet-like morphology and Ni₃S₂ appears as nanoparticles (Fig. S2). As shown in Fig. S3a, Cu–Ni₃S₂ exhibits an intercalated structure composed of flakes and nanospheres. Its flake-like structure is similar to that observed in the SEM image of Cu–NF, confirming the successful load of Cu on the surface of Ni₃S₂. After electrochemical treatment, the activated Cu–Ni₃S₂ reveals a network of loose nanospheres and smaller particle sizes which may expose more active sites in the HMFOR process (Fig. S3b). As shown in Fig. 1b and c, TEM images of activated Cu–Ni₃S₂ exhibit 0.28 nm lattice fringes, corresponding to the (110) crystal plane of Ni₃S₂.¹⁶ The energy-dispersive X-ray spectroscopy (EDS) shows that Cu, Ni, S, and O elements are uniformly distributed on the activated Cu–Ni₃S₂ surface (Fig. 1d–h and Fig. S4).

Fig. 1 (a) Schematic illustration of the synthesis process of Cu–Ni₃S₂. (b and c) TEM images of activated Cu–Ni₃S₂. (d–h) EDS images of activated Cu–Ni₃S₂.

The crystal structure of Cu–Ni₃S₂ was characterized by X-ray diffraction (XRD). In Fig. S5a, the prepared Cu–Ni₃S₂ shows the characteristic diffraction peaks of Cu and Ni₃S₂. The diffraction peaks of Cu are located at 43.97, 51.14, and 74.65°, corresponding to (111), (200), and (220) crystal faces, respectively (PDF # 04-0836), while the diffraction peaks of Ni₃S₂ (PDF # 44-1418) at 21.7, 31.1, 37.9, 38.3, 44.4, 49.8, 50.2 and 55.2° correspond to the (101), (110), (003), (021), (202), (113), (211), and (122) crystal faces. Additionally, the three strong diffraction peaks located at 44.5, 51.8 and 76.37° can be attributed to the Ni (PDF #04-0850) from the NF substrate.^27–30 After electrochemical activation (Fig. S5b), the weak intensity of activated Cu–Ni₃S₂ peaks indicates the decreased crystallinity during activation, which might promote the conversion of hydroxyl oxides and consequently enhance catalytic activity.³¹ Meanwhile, the persistence of major diffraction peaks demonstrates their stability under high voltage conditions. Raman spectroscopy was used to further investigate the structure of Cu–Ni₃S₂ (Fig. S6). The vibration modes of Ni–S in 150–400 cm⁻¹ are found in both Cu–Ni₃S₂ and activated Cu–Ni₃S₂, while broad characteristic peaks in the range of 400–550 cm⁻¹ can be assigned to the active amorphous NiOOH.³² For the activated Cu–Ni₃S₂, the characteristic peaks in the range of 600–700 cm⁻¹ correspond to the typical vibrations of CuOOH species, confirming the successful formation of the active CuOOH species.³³ Therefore, Ni and Cu are successfully induced to form high-valence Ni³⁺ and Cu³⁺, which serve as the primary active sites for the HMFOR.

X-ray photoelectron spectroscopy (XPS) was used to characterize the valence states of Cu–Ni₃S₂, and its full-scan XPS spectra before and after activation are shown in Fig. S7 and S8. The high-resolution XPS spectrum of Cu 2p is shown in Fig. 2a, the binding energies of Cu 2p_3/2 and Cu 2p_1/2 of Cu are located at 932.4 eV and 952.3 eV and the other two peaks at 934.6 eV and 954.4 eV are attributed to Cu 2p_3/2 and Cu 2p_1/2 of Cu²⁺,^34–37 while the remaining peaks are satellite peaks. After electrochemical activation (Fig. 2b), both the Cu²⁺ 2p_3/2 and Cu²⁺ 2p_1/2 peaks are located at 934 eV and 955.1 eV, which is highly consistent with the initial Cu–Ni₃S₂. The peaks at higher binding energies, 935.2 eV and 956.51 eV, correspond to Cu³⁺ 2p_3/2 and Cu³⁺ 2p_1/2, respectively, indicating the presence of CuOOH species.³⁸ The high-resolution XPS spectrum of Ni 2p before activation is presented in Fig. 2c, in which the characteristic peaks of Ni²⁺ are located at 855.3 (Ni²⁺ 2p_3/2) and 873.1 eV (Ni²⁺ 2p_1/2) and the characteristic peaks of Ni³⁺ are located at 857.5 (Ni³⁺ 2p_3/2) and 875.6 eV (Ni³⁺ 2p_1/2).³⁹ The characteristic 2p_3/2 peak of Ni²⁺ in activated Cu–Ni₃S₂ shifts toward higher binding energies by approximately 0.6 eV, confirming the existence of strong electronic interactions between Ni and Cu and that the introduction of Cu species promotes the formation of higher-valent Ni species.⁴⁰ In the O 1s XPS spectrum of activated Cu–Ni₃S₂ (Fig. S9), the three peaks fitted at 528.5, 530.8, and 532.6 eV correspond to lattice oxygen (O_L), oxygen vacancies (O_V), and chemisorbed oxygen (O_C), respectively.^41,42 It can be observed that O_V dominate and serve as direct adsorption points for reactant molecules. Furthermore, the increased number of adsorption sites enhances HMF adsorption, thereby improving catalytic performance.⁴³

Fig. 2 XPS spectra of Cu 2p for Cu–Ni₃S₂ before (a) and after (b) activation. XPS spectra of Ni 2p 1s for Cu–Ni₃S₂ before (c) and after (d) activation.

Cyclic voltammetry (CV) was used to evaluate the electrochemical activation process of the catalyst in 1 M KOH. Fig. 3a shows the first CV curve of Cu–NF, which includes four sets of redox peaks. The oxidation peak (a) corresponds to Cu⁰ → Cu⁺, the oxidation peak (b) corresponds to Cu⁰/Cu⁺ → Cu²⁺, and the oxidation peak (c) is related to the oxidation of Cu²⁺ → Cu³⁺ and Ni²⁺ → Ni³⁺. The reduction peaks (d, f and g) correspond to Cu³⁺ → Cu²⁺, Ni³⁺ → Ni²⁺, Cu²⁺ → Cu⁰/Cu⁺, and Cu⁺ → Cu⁰, respectively.^44,45Fig. 3b shows the second CV curve of Cu–NF, and the oxidation peaks of Cu⁰/Cu⁺ → Cu²⁺ become weaker, while the oxidation peaks of Ni become stronger. As shown in Fig. 3c, the oxidation peak (a) and reduction peak (b) of Ni in Ni₃S₂ exhibit higher intensity than those in Cu–NF, indicating that S accelerates the conversion of Ni to higher oxidation states. The second CV curve of Ni₃S₂ is shown in Fig. 3d, and the oxidation peak of Ni becomes stronger.³² The first CV curve of Cu–Ni₃S₂ is shown in Fig. 3e, and its oxidation and reduction peaks are similar to those of Cu–NF; the oxidation peak of (a) is ascribed to Cu⁰ → Cu⁺, the strong oxidation peak of (b) is from Cu⁰/Cu⁺ → Cu²⁺, while the oxidation peaks of (c) are due to Ni²⁺ → Ni³⁺ and Cu²⁺ → Cu³⁺. The reduction peaks of (d) originate from Ni³⁺ → Ni²⁺ and Cu³⁺ → Cu²⁺, the reduction peak of (e) stems from Cu²⁺ → Cu⁰/Cu⁺, and the reduction peak of (f) corresponds to Cu⁺ → Cu⁰. From the second CV curve of Cu–Ni₃S₂ (Fig. 3f), it can be observed that the redox peaks of Cu and Ni become stronger after electrochemical activation. The Ni oxidation peaks (c) of both Cu–Ni₃S₂ and Ni₃S₂ exhibit stronger redox peaks, indicating that they are more readily oxidized to higher-valent NiOOH active species, compared to Cu–NF.¹⁵ Besides, the Ni oxidation peak of Cu–Ni₃S₂ is broader than that of Ni₃S₂, but its peak intensity has decreased. The oxidation peak of Ni₃S₂ exhibits a peak value of 53.7 mA cm⁻², while that of Cu–Ni₃S₂ decreases to 46.62 mA cm⁻². The shape change of the Ni oxidation peak is because the oxidation peak of Cu (Cu²⁺ → Cu³⁺) overlaps with the oxidation peak of Ni (Ni²⁺ → Ni³⁺), and the Cu oxidation peak broadens the Ni oxidation peak. Moreover, the Cu suppresses the OER and reduces the height of the Ni oxidation peaks, which can result in higher FE of the HMFOR process.

Fig. 3 The first (a) and second (b) cycles of CV over Cu–NF in 1 M KOH. The first (c) and second (d) cycles of CV over Ni₃S₂ in 1 M KOH. The first (e) and second (f) cycles of CV over Cu–Ni₃S₂ in 1 M KOH.

To systematically evaluate the electrochemical performance of the HMFOR, linear sweep voltammetry (LSV) analysis was performed on Cu–NF, Ni₃S₂ and Cu–Ni₃S₂.⁴⁶ The LSV curves of Cu–Ni₃S₂ in 1 M KOH and 10 mM HMF are illustrated in Fig. 4a. The oxidation peak within the 0.8V_RHE–1.0V_RHE range could correspond to the oxidation of Cu²⁺ to Cu³⁺, while the peak located near the 1.4V_RHE range belongs to the oxidation of Ni²⁺.^47,48 During the second LSV curve, the oxidation peak from Ni²⁺ to Ni³⁺ becomes stronger, which indicates that more active NiOOH species have been generated on the surface of Cu–Ni₃S₂. In comparison, the second LSV curve of Ni₃S₂ (Fig. S10a) reveals a weaker Ni²⁺ to Ni³⁺ oxidation peak compared to that of Cu–Ni₃S₂, indicating that Ni²⁺ in Ni₃S₂ is less activated to higher oxidation states. As shown in Fig. S10b, the oxidation peak of Cu appears in the first LSV scan of Cu–NF but disappears in the second LSV scan, which is similar to that of Cu–Ni₃S₂ (Fig. 4a).⁴⁰ The effect of different Cu loading contents on the performance of the HMFOR was investigated by regulating the electrodeposition time (Fig. S11). The results show that the highest current density is obtained when the deposition time is 180 s. The effect of Cu on Ni₃S₂ for the HMFOR was studied (Fig. S12), and activated Cu–Ni₃S₂ exhibits higher curves than activated Ni₃S₂ and activated Cu–NF, demonstrating superior HMFOR performance. Meanwhile, the current density of activated Cu–Ni₃S₂ (56.85 mA cm⁻²) is approximately three times higher than that of activated Cu–NF (18.61 mA cm⁻²) and significantly exceeds that of activated Ni₃S₂ (36.85 mA cm⁻²) at 1.50V_RHE (Fig. 4b). The above results are primarily attributed to the synergistic interaction between Cu and Ni, which promotes the formation of highly reactive Ni³⁺/Cu³⁺ species. As shown in Fig. 4c, the Tafel slope of activated Cu–Ni₃S₂ (108.01 mV dec⁻¹) is significantly smaller than that of activated Ni₃S₂ (181.38 mV dec⁻¹) and activated Cu–NF (198.18 mV dec⁻¹), which indicates that activated Cu–Ni₃S₂ exhibits excellent HMFOR kinetics. The electrochemical double layer capacitance (C_dl) was determined by quantitatively analyzing the non-Faraday zone in the CV curve (Fig. S13). As shown in Fig. 4d, the results clearly show that the activated Cu–Ni₃S₂ has a higher HMFOR active surface area, compared to the activated Cu–NF and Ni₃S₂. The catalytic activity of the active species Cu³⁺/Ni³⁺ was further explored by LSV (Fig. 4e). The results indicate that HMF concentration correlates positively with the expansion of the electrochemical voltage window and the increase of current density. Furthermore, the linear relationship between current density and HMF concentration indicates that the kinetics of HMF oxidation (HMFOR) on the active Cu–Ni₃S₂ is diffusion-limited (Fig. 4f).

Fig. 4 (a) LSV curves of Cu–Ni₃S₂ without and with 10 mM HMF. (b) HMFOR current densities of activated Cu–NF, Ni₃S₂, and Cu–Ni₃S₂ at 1.50V_RHE. (c) Tafel slopes of activated Cu–NF, Ni₃S₂, and Cu–Ni₃S₂. (d) Capacitive current densities of activated Cu–NF, Ni₃S₂, and Cu–Ni₃S₂. (e) LSV curves of activated Cu–Ni₃S₂ in the presence of HMF with different concentrations (scan rate: 5 mV s⁻¹). (f) The current density vs. concentration of HMF.

Electrochemical impedance spectroscopy (EIS) was used to test the interfacial electrochemical behavior at different voltages to further elucidate the difference in reaction kinetics between the OER and the HMFOR.^49,50 The low-frequency region (10⁻¹–10¹ Hz) correlates with non-uniform charge distribution, indicating the emergence of oxides at the electrode interface. In the EIS spectra without HMF (Fig. S14), a peak in the low-frequency region is observed at 1.6V_RHE, indicating the occurrence of the OER. A new peak appears at 1.4V_RHE for Ni₃S₂ and Cu–NF after HMF addition, indicating the occurrence of the HMFOR (Fig. 5a and S15). Meanwhile, as shown in Fig. 5b, the Cu–Ni₃S₂ exhibits a similar trend to Ni₃S₂. Compared to Ni₃S₂ and Cu–NF, the HMFOR in Cu–Ni₃S₂ occurs earlier. This result is consistent with the LSV curves of the HMFOR (Fig. S12). Furthermore, Cu–Ni₃S₂ exhibits a faster decay trend in the OER, indicating that its superior HMFOR performance is achieved with suppressed OER at high voltage.

Fig. 5 Analysis of the interfacial electrochemical behavior of synthetic samples. EIS spectra of (a) Ni₃S₂ and (b) Cu–Ni₃S₂ under different potentials in 10 mM HMF. (c) OCPT with and without 10 mM HMF. (d) Six cycles of OCPT of Cu–Ni₃S₂ in 10 mM HMF.

Open-circuit potential vs. time (OCPT) was used to investigate the self-healing properties of Cu–Ni₃S₂. As shown in Fig. 5c, the initial OCP of 1.45V_RHE in 1 M KOH is reduced to 1.24V_RHE after 10.5 h, which is related to the oxidation potential of the active species of Cu³⁺/Ni³⁺. With the addition of 10 mM HMF, the OCP decreases to 0.93V_RHE within 600 s, which is consistent with the oxidation potential of Cu²⁺. Importantly, the six consecutive OCPT cycles under the HMFOR demonstrate reproducible potential recovery, directly evidencing the dynamic redox equilibrium between Cu²⁺ ⇌ Cu³⁺ and Ni²⁺ ⇌ Ni³⁺ pairs (Fig. 5d).⁵¹ This persistent regeneration capability suggests a synergistic charge-compensation mechanism, in which the electron-deficient Ni³⁺ centers can promote Cu²⁺ oxidation, while Cu³⁺ stabilizes neighboring Ni³⁺ through intermetallic charge transfer, collectively enabling robust self-repair functionality.

Additionally, the schematic diagram in Fig. 6 schematically proposes the oxidation mechanism of HMF over Cu–Ni₃S₂. The HMFOR mainly proceeds via a two-step oxygen transfer mechanism.³⁸ XPS and Raman spectroscopy reveal that Cu–Ni₃S₂ undergoes electrochemical oxidation under alkaline conditions, forming highly oxidized CuOOH and NiOOH species. Subsequently, OCPT measurements and other electrochemical tests demonstrate that these CuOOH and NiOOH active species could oxidize HMF to the corresponding products, while Cu³⁺/Ni³⁺ are reduced to Cu²⁺/Ni²⁺. This reduction step completes the catalytic cycle involving the metal centers. This mechanism highlights the essential role of the electrochemically formed CuOOH/NiOOH species as the active oxidizing agents. The catalyst effectively serves as a redox mediator, cycling between its oxidized (Cu³⁺/Ni³⁺ in the oxyhydroxide form) and reduced (Cu²⁺/Ni²⁺) states. Therefore, the efficiency of the entire HMFOR process is closely related to the stability and reactivity of these high-valent hydroxyl oxide intermediates formed on Cu–Ni₃S₂.

Fig. 6 The schematic illustration of HMF oxidation for the activated Cu–Ni₃S₂ catalysts.

As shown in Fig. 7a, the study of the reaction mechanism of HMF reveals that there are two possible main pathways in the HMFOR.⁵² In the first pathway, the hydroxyl functional group in the HMF molecule undergoes selective oxidation and is converted into an aldehyde group to form HMFCA with a bis-aldehyde structure.⁵³ The parallel second pathway involves the oxidation of the aldehyde group to produce DFF containing both hydroxyl and carboxyl functional groups through a carboxylation process. It is noteworthy that both primary oxidation products (HMFCA and DFF) are capable of generating the co-intermediate FFCA, which carries both aldehyde and carboxyl functional groups through successive oxidation reactions. Finally, FFCA is further fully oxidized to form the end product FDCA with two carboxyl groups. The temporal evolution of substrate (HMF) and product concentrations (HMFCA, DFF, FFCA, and FDCA) was quantitatively monitored by HPLC, with calibration curves for these analytes presented in Fig. S16–S20. To determine the optimal oxidation potential of activated Cu–Ni₃S₂ at high voltage, the electrochemical experiments were conducted within the range of 1.5V_RHE to 1.6V_RHE. As the reaction progressed, the signal intensity of HMF gradually decreased, while the signal intensity of FDCA progressively increased (Fig. S21a). During the reaction, only trace amounts of the intermediates HMFCA and FFCA were detected without DFF (Fig. 7b), so the HMF oxidation pathway of activated Cu–Ni₃S₂ in 1 M KOH follows Path 1. As shown in Fig. S21b, the current density also gradually decreases with the progress of the reaction, which indicates that HMF has gradually converted. As shown in Fig. S22 and Fig. 7c, due to the lower current density (1.5V_RHE), the complete oxidation of HMF takes a longer time (3.5 h). The lower FE is attributed to the partial degradation of HMF, which undergoes slow oxidation to form intermediate products. As the voltage increases to 1.55V_RHE, the highest FE of 100% is obtained. However, when the voltage increases to 1.6V_RHE, the competitive OER intensifies, leading to a decrease in FE (Fig. S23 and Fig. 7d). Additionally, the FDCA yield of activated Cu–Ni₃S₂ is compared at different bias voltages. The results show that the FDCA yield at 1.55V_RHE (100%) is also significantly higher than those at 1.5V_RHE (91%) and 1.6V_RHE (86.9%) (Fig. 7e). Consequently, the optimal oxidation potential for the HMFOR of activated Cu–Ni₃S₂ is 1.55V_RHE.

Fig. 7 (a) Two possible pathways of the HMFOR. Concentration of HMF, FDCA, HMFCA, and FFCA, and total charge vs. time over the activated Cu–Ni₃S₂ at (b) 1.55V_RHE, (c) 1.5V_RHE, and (d) 1.6V_RHE (1 M KOH, 10 mM HMF). (e) Radar plot of activated Cu–Ni₃S₂ at different voltages (1 M KOH and 10 mM HMF). (f) Radar diagram of activated Cu–NF, Ni₃S₂, and Cu–Ni₃S₂, at 1.55V_RHE (1 M KOH, 10 mM HMF). (g) Comparison of FE at the optimum bias voltage for activated Cu–Ni₃S₂ with previously reported electrocatalysts.

To investigate the effect of Cu on the HMF conversion of Ni₃S₂, the FDCA yields of activated Cu–Ni₃S₂ were compared with Cu–NF and Ni₃S₂ at 1.55V_RHE. The results demonstrate that activated Cu–Ni₃S₂ exhibits outstanding HMF conversion performance (HMF conversion: 100%, FDCA yield: 100%, FE: 100%), outperforming Ni₃S₂ (HMF conversion: 97.5%, FDCA yield: 94.8%, FE: 94.8%) and Cu–NF (HMF conversion: 100%, FDCA yield: 88.9%, FE: 88.9%) (Fig. S24–S27 and Fig. 7f). The above findings indicate that Ni₃S₂ and Cu–NF exhibit poor HMFOR catalytic activity. Ni₃S₂ exhibits severe competition in the OER at 1.55V_RHE, resulting in lower FE. For Cu–NF, the reaction persists for a long time (5 h) to achieve 100% HMF conversion, meanwhile the FE is low. Therefore, Cu enables the HMFOR of Ni₃S₂ at high voltage. To investigate the stability of activated Cu–Ni₃S₂ in the HMFOR, cyclic stability experiments were carried out. After five consecutive cycling experiments, the HMF conversion and FE remain virtually unchanged, and FDCA still reaches 98%. Post-electrolysis inductively coupled plasma (ICP) analysis of the electrolyte indicated low leaching (Cu 3.07 mg L⁻¹; Ni 0.089 mg L⁻¹) (Table S1), confirming stable composition. These demonstrate that activated Cu–Ni₃S₂ is a catalyst with outstanding stability (Fig. S28 and S29). As shown in Table S2 and Fig. 7g, the activated Cu–Ni₃S₂ exhibits superior FE at high voltage, compared with the previously reported Ni₃S₂ catalysts. Therefore, these results indicate that activated Cu–Ni₃S₂ demonstrates excellent stability and performance in the HMFOR at high voltage.

3. Conclusions

In summary, Cu–Ni₃S₂ was successfully fabricated for the HMFOR at high voltage. The electrochemically induced dual active species (Cu³⁺/Ni³⁺) demonstrated an excellent performance in the HMFOR, achieving complete HMF conversion, 100% FDCA yield, and 100% FE. OCPT analysis and cycling stability tests confirm the remarkable catalytic stability of the Cu³⁺/Ni³⁺ under 1.55V_RHE. Notably, the retention rate of FDCA yield can still reach 98% after five consecutive reaction cycles, highlighting the superior durability of the catalyst. Therefore, this study provides an in-depth understanding of the role of Cu in the Ni₃S₂ matrix, offering a new approach for designing high-voltage-resistant catalytic systems.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data are available from the corresponding author on reasonable request.

Supplementary information (SI) is available. Detailed characterizations, SEM, XRD, Raman images, electrochemical measurements, and HPLC analysis. See DOI: https://doi.org/10.1039/d6qi00021e.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (22578184), the Natural Science Foundation of Jiangsu Province (BK20251902), the College Student Innovation Training Program and the Open Project of Key Laboratory of Jiangxi University for Functional Materials Chemistry (FMC25701), and the Open Project of the Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education).

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Footnote

† These authors contributed equally to this work.

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