Bio-inspired proton relay for promoting continuous 5-hydroxymethylfurfural electrooxidation in a flowing system

Abstract

Electrooxidation of 5-hydroxymethylfurfural (HMF) to produce the platform molecule 2,5-furandicarboxylic acid (FDCA) provides a promising approach for biomass upgrading and green hydrogen production. However, the slow reaction kinetics and poor stability of the anode catalyst hinder continuous FDCA production in flowing systems at industrial current densities (>200 mA cm⁻²). Herein, a ligand-modified catalyst, Ni(OH)₂–TPA (TPA: terephthalic acid), is synthesized for efficient HMF oxidation, wherein the uncoordinated carboxylate functions as the proton relay center, significantly enhancing oxidation performances. The current density is increased 16-fold compared to that of pure Ni(OH)₂, and the faradaic efficiency of FDCA reaches 96.9 ± 0.2% at even 1000 mA cm⁻². Consequently, an anion exchange membrane electrolyzer (25 cm²) is constructed that shows a current of 10.3 A at 1.80 V. The system operates stably for over 240 hours at 7500 mA, producing hectogram-level FDCA continuously with an overall productivity of 2.85 kg m⁻² h⁻¹. These results offer insightful strategies for designing catalysts and fabricating electrolyzers for industrial applications.

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Broader context

Production of FDCA through HMF electrooxidation is considered a sustainable and promising method to synthesize bio-based plastic polyethylene furanoate (PEF). Previous studies have focused on developing effective electrocatalysts for HMF oxidation in a three-electrode system but they are yet to be investigated under industrial-relevant current density (>200 mA cm⁻²) and durability (>200 hours) conditions, particularly for continuous FDCA production in flowing systems. Due to the multiple-proton transfer process during HMF oxidation, the reaction kinetics are greatly hampered. Here, we have constructed a ligand-modified catalyst, Ni(OH)₂–TPA, for continuous FDCA production in an AEM-based electrolyzer under industrial conditions. Ni(OH)₂–TPA significantly promoted HMF oxidation kinetics by accelerating the proton relay process, exhibiting a current density of 850 mA cm⁻² in a three-electrode system and a current of 10.3 A in an AEM electrolyzer (25 cm²). The electrolyzer has been proven to run steadily for over 240 hours at 7500 mA, showing continuous FDCA production with a productivity of 2.85 kg m⁻² h⁻¹. The excellent activity and stability of this AEM electrolyzer demonstrate its potential for industrial-scale implementation, providing a robust foundation for future catalyst design and electrolyzer development for biomass upgrading.

Introduction

In 2023, the global plastics market attained a valuation of 624.8 billion USD.¹ Within the market, bio-based polymers are getting increasing attention due to their relevance in sustainable development. Polyethylene furanoate (PEF), synthesized from 2,5-furandicarboxylic acid (FDCA), is a promising bio-based polymer with excellent degradability, thermal properties, and mechanical strength.^2,3 The production of FDCA through an electrocatalytic 5-hydroxymethylfurfural oxidation reaction (HMFOR) is considered an eco-friendly route, as it operates under ambient atmosphere and benign temperature conditions with H₂O as the green oxygen source (Note S1, ESI†).^4,5

Despite this, constructing a HMF oxidation electrolyzer capable of performing at industrial-related current densities still lacks efficient catalysts.^6,7 Catalysts based on Ni, Co, and Cu have been broadly investigated for HMF oxidation but failed to meet the demands of a current density of >200 mA cm⁻² and a durability of >200 hours, particularly for flowing systems.⁸ The potential required typically exceeds 1.6 V versus reversible hydrogen electrode (RHE) in a three-electrode configuration and over 2.0 V in an anion exchange membrane (AEM)-based setup.^9–11 In an HMFOR electrolyzer with high current densities, the generated proton may create a microenvironment with low local pH at the anode–membrane interface, complicating the evaluation of the electrolyzer's overall performance.¹² Hence, developing a robust and high-performance catalyst that features rapid HMF oxidation kinetics is of great importance. Strategies such as element doping, defect engineering, and morphology regulation have been employed to enhance HMF oxidation kinetics, but the improvements are quite limited.¹³ Achieving superior performance requires operating at a low working potential (<2.0 V), as higher potentials may lead to catalyst overoxidation and dissolution of the metallic center. Achieving a high faradaic efficiency (FE) of FDCA can prevent the generation of bubbles by the competitive oxygen evolution reaction (OER), which prevents the catalyst from detaching from the substrate.¹⁴ Therefore, deep insights into the reaction process are necessary to design a practically viable catalyst that meets these requirements.

HMF oxidation involves a multiple-proton transfer process (Scheme 1). Under strong alkaline conditions (pH > 13), the aldehyde group (–CHO) of HMF is initially oxidized to carboxylic acid (–COOH) by transferring one proton in the presence of two OH⁻ ions, making 5-hydroxymethyl carboxylic acid (HMFCA) the first intermediate.¹⁵ Subsequently, 5-formyl-2-furancarboxylic acid (FFCA) is formed by dehydrogenating the hydroxymethyl (–CH₂OH) group of HMFCA to an aldehyde group involving a two-proton transfer and losing two electrons. Finally, the remaining –CHO is oxidized to –COOH, thereby producing FDCA. Due to the four-proton transfer process, the overall oxidation reactivity would be greatly restricted, requiring excess potentials to drive HMF oxidation with high productivity. In biocatalytic systems, liver alcohol dehydrogenase (LADH) effectively facilitates alcohol dehydrogenation by accelerating proton transfer.^16–18 As depicted in Scheme 1a, the proton generated by the hydroxyl group of the substrate (benzyl alcohol) is transferred via oxygen/nitrogen in the residue of Ser-48, cofactor (NAD⁺), and His-51 through the proton transfer chains (PT1, PT2, and PT3). Here, the oxygen center acts as a proton relay bridge, effectively shuttling the proton and thus enhancing reaction kinetics.

Scheme 1 (a) 3D structure of LADH (PDB: 1AXG) and the proton transfer process during dehydrogenation. (b) Reaction route of HMF oxidation to FDCA under strong alkaline conditions (left) and bio-inspired proton relay through the TPA ligand of Ni(OH)₂–TPA (right). HMFCA: 5-hydroxymethylfurancarboxylic acid; FFCA: 5-formyl-2-furancarboxylic acid.

Inspired by this, to construct an analogical proton transfer bridge, we have successfully synthesized a ligand-modified Ni(OH)₂ catalyst (Ni(OH)₂–TPA, TPA: terephthalic acid) for efficient HMF oxidation (Scheme 1b). In situ infrared and Raman spectroscopy analyses have confirmed that the TPA ligand works as the proton relay center, significantly improving the reaction kinetics of HMF oxidation. A current density of 850 mA cm⁻² is obtained at 1.55 V vs. RHE, and the FE of FDCA reaches 96.9 ± 0.2% at even 1000 mA cm⁻². Ni(OH)₂–TPA runs stably for 200 hours in a three-electrode setup and performed well in an AEM-based electrolyzer. The electrolyzer exhibits a current of 10.3 A at 1.80 V and operates steadily for over 240 hours at 7500 mA, enabling continuous FDCA production at the hectogram level. Our findings provide an efficient strategy for designing HMF oxidation catalysts through bio-inspired ligand modification and demonstrate the practicality in industrial-related scenarios.

Results and discussion

Synthesis and structural characterization

Ni(OH)₂–TPA supported on Ni foam (NF) is prepared using a facile two-step procedure (see details in Methods, Fig. S1, ESI†).¹⁹ Ni(OH)₂ nanoflakes are first synthesized via a hydrothermal method at 120 °C, followed by treatment with TPA in N,N-dimethylformamide (DMF) solution, and the obtained catalyst is denoted as Ni(OH)₂–TPA. Scanning electron microscopy (SEM) images reveal that Ni(OH)₂–TPA consists of nanoflakes with a thickness of 40–50 nm, uniformly and vertically distributed on NF (Fig. 1a and Fig. S2–S3, ESI†). The X-ray diffraction (XRD) pattern is obtained to identify the crystal structure of the catalyst (Fig. 1b and Fig. S4, ESI†). The peaks at 19.2°, 33.2°, and 38.6° correspond to the (001), (100), and (011) crystallographic planes of standard Ni(OH)₂ (PDF#73-1520), respectively. Additionally, Ni(OH)₂–TPA exhibits a new peak at 8.8°, indicating a larger interlayer spacing, in line with the size of the TPA molecule. This suggests that TPA is intercalated into the Ni(OH)₂ layers. The ex situ Raman spectrum of Ni(OH)₂–TPA shows doublets at 1426 and 1612 cm⁻¹ (Fig. 1c), which are associated with the in- and out-of-phase stretching modes of the carboxylate group (–COO⁻) as observed for sodium terephthalate (Na₂TPA).²⁰ A small peak at 1636 cm⁻¹ is attributed to the carboxyl groups (–COOH). The Fourier transform infrared (FT-IR) spectra of pristine Ni(OH)₂–TPA exhibit adsorption peaks of ν_as(–COO⁻) and ν_s(–COO⁻) coordinated to the metal centers at 1375 and 1578 cm⁻¹, respectively (Fig. 1d).^21,22 The signals of –COOH that are visible at 1425 and 1658 cm⁻¹ vanish after activation in the KOH solution.²³ These results demonstrate that TPA is intercalated into the interlayer of Ni(OH)₂, with partial carboxylate groups coordinated with the metal ions (Fig. S5, ESI†).

Fig. 1 (a) SEM image of Ni(OH)₂–TPA. Inset: magnified image. (b) XRD patterns of Ni(OH)₂–TPA and Ni(OH)₂. (c) Ex situ Raman spectra. Na₂TPA: sodium terephthalate; H₂TPA: terephthalic acid. (d) Ex situ FT-IR spectra of H₂TPA, Na₂TPA, pristine Ni(OH)₂–TPA, and activated Ni(OH)₂–TPA.

Electrocatalytic performance evaluation

The intrinsic redox properties of Ni(OH)₂–TPA and Ni(OH)₂ are first investigated in 1.0 M KOH solution. From cyclic voltammetry (CV) curves shown in Fig. S6 (ESI†), Ni(OH)₂–TPA exhibits a higher oxidation current corresponding to Ni²⁺ to Ni³⁺ transition, indicating the exposure of more active Ni sites in Ni(OH)₂–TPA. CV plots toward the OER of these two catalysts are recorded in Fig. S7 (ESI†). Ni(OH)₂–TPA displays an earlier onset potential (at 1 mA cm⁻²) of 1.44 V vs. RHE, 70 mV lower than that of Ni(OH)₂. Ni(OH)₂–TPA also has an improved OER activity with an overpotential of 271 mV at 10 mA cm⁻², 70 mV lower than that of Ni(OH)₂ (Fig. S8, ESI†), suggesting enhanced activity through the TPA ligand. To accurately assess the performance of HMF oxidation, a quasi-steady-state linear sweep voltammetry (QS-LSV) study is conducted based on the equilibrated current density under a given potential (Fig. 2a).^24,25 As shown, the current density greatly intensifies in the same potential window of the OER when HMF is introduced, and has reached 674 ± 34 mA cm⁻² at 1.50 V for Ni(OH)₂–TPA, 16-fold exceeding 41 ± 3 mA cm⁻² for Ni(OH)₂. The catalyst response for HMF oxidation is studied with increased potentials, and the results are analyzed using high-performance liquid chromatography (HPLC, Fig. S9, ESI†). Ni(OH)₂–TPA exhibits an FDCA yield of over 97.3 ± 0.7%, and faradaic efficiency (FE) of over 96.8 ± 0.9% within the given potential window (Fig. 2b and Fig. S10, ESI†). While for Ni(OH)₂, the yield merely reaches 16.8 ± 2.9% at 1.40 V and limitedly increases to 32.7 ± 3.3% at 1.50 V. HMF is rapidly converted to FDCA as the primary product, with HMFCA and FFCA as the intermediates within the first 25 min (Fig. 2c). As the reaction goes by, HMF and the corresponding intermediates are fully oxidized to FDCA within 100 min (Fig. S11, ESI†). The FDCA concentration reflects nearly 100% carbon balance, and the practical charge accumulated is consistent with the theoretical value, reflecting high FEs of FDCA as well (Fig. 2c and Fig. S12, ESI†).

Fig. 2 (a) QS-LSV plots of Ni(OH)₂–TPA and Ni(OH)₂ in 1.0 M KOH with 100 mM HMF. (b) FDCA yield at elevated potentials. Reaction time: 100 min. (c) Evolution of concentrations and accumulated charges over time at 1.45 V. (d) FE of the oxidation products (HMFCA, FFCA, and FDCA) at varied current densities with 100 C charge passed. (e) Stability evaluation of Ni(OH)₂–TPA for six consecutive batches with 100 mM HMF (each batch lasting 100 min). (f) CP test of Ni(OH)₂–TPA at a current density of 200 mA cm⁻².

To further test the applicability of Ni(OH)₂–TPA under industrial current densities for HMF oxidation, the FEs were evaluated at elevated current densities and are shown in Fig. 2d. The FEs of all oxidation products (HMFCA, FFCA, and FDCA) remain at 98.9 ± 1.6% at 100 mA cm⁻² and 96.9 ± 0.2% at even 1000 mA cm⁻² for Ni(OH)₂–TPA, which presents high reaction specificity for being applied in practical electrolyzers (Fig. S13, ESI†). However, the FE for Ni(OH)₂ is 96.3 ± 1.1% at 100 mA cm⁻² and drastically decays to 61.8 ± 2.6% at 1,000 mA cm⁻² due to the vigorous OER process. In the presence of 100 mM HMF, the performance of Ni(OH)₂–TPA remains consistent throughout six consecutive cycles, where yields and FEs of FDCA are maintained at ∼100% (Fig. 2e and Fig. S14, 15, ESI†). The current density decays from over 500 mA cm⁻² to nearly zero with the complete conversion of HMF. The high FEs of FDCA prevent the competition of the OER and eliminate Ni(OH)₂–TPA detachment from the substrate by O₂ bubbles. In Fig. 2f, the chronopotentiometry (CP) curve obtained at 200 mA cm⁻² shows a negligible change for over 200 hours (Fig. S16, ESI†). The nanoflakes are well preserved after the test, indicating the superior stability of Ni(OH)₂–TPA as well (Fig. S17, ESI†).

The electrochemical surface area (ECSA) test is carried out through double-layer capacity (C_dl) to analyze the intrinsic activity of the catalysts. C_dl of Ni(OH)₂–TPA in KOH solution is measured to be 5.25 ± 0.49 mF cm⁻², surpassing 2.62 ± 0.15 mF cm⁻² of Ni(OH)₂ (Fig. S18, ESI†). After introducing HMF, the C_dl values of both catalysts remain nearly constant, suggesting that the active sites for the OER could also be active for HMF oxidation.²⁶ LSV plots normalized through the ECSA are presented in Fig. 3a, where Ni(OH)₂–TPA outperforms Ni(OH)₂ as well. Meanwhile, the activity normalized through the area of the Ni³⁺ reduction peak also confirms its superior intrinsic performance (Fig. S19, ESI†). As shown, Ni sites serve as catalytically active sites for HMF oxidation. Thus, to roughly compare the number of these sites, catalysts are first oxidized at 1.45 V long enough to convert all Ni²⁺ to Ni³⁺. The charge passing through the electrode is then recorded by applying a reducing potential at 1.0 V to form Ni(OH)₂ from NiOOH. As shown in Fig. 3b, Ni(OH)₂–TPA possesses more passed charges (−0.83 C) than Ni(OH)₂ (−0.13 C), denoting that the TPA ligand not only merely brings the exposure of active Ni sites as uncovered by the ECSA test, but may also enhance the catalytic efficiency of specific sites for HMF oxidation. This is also validated by the normalization of the current density through the passed charge (Fig. S20, ESI†).

Fig. 3 (a) ECSA-normalized QS-LSV plots. (b) Charge passed through the electrode for NiOOH reduction. (c) Steady state Tafel slopes. Each point represents the steady state potential lasting over 60 seconds at a given current density. The lighter color represents the error band. (d) Evolution of OCP values after injecting 100 mM HMF into 1.0 M KOH (black arrow). Operando EIS analysis for (e) Ni(OH)₂–TPA and (f) Ni(OH)₂ in 1.0 M KOH with 100 mM HMF.

A steady state Tafel slope analysis is further applied to analyze the reaction kinetics during the HMFOR (Fig. 3c). Ni(OH)₂ presents a Tafel slope of 84.8 mV dec⁻¹ under a low current window, which increases to 189.7 mV dec⁻¹ at higher current densities, consistent with the LSV results (Fig. 2a). As the external potential is strong enough to generate NiOOH rapidly from Ni(OH)₂, the reactivity for HMF oxidation of NiOOH primarily influences the reaction rate, as evidenced by the Ni²⁺/Ni³⁺-like oxidation peak in Fig. 2a and 3a.²⁷ In contrast, the Tafel slope of Ni(OH)₂–TPA is calculated to be 30.3 ± 13.1 mV dec⁻¹ throughout the given current range, as confirmed by the almost linear increase in LSV plots. This potential-dependent process directly determines the reactivity by the NiOOH generation rate driven by the increased potential.²⁸ This is also reflected by the fast decrease of open-circuit potential (OCP) values, where the oxidized Ni(OH)₂–TPA is reduced more rapidly than Ni(OH)₂ upon HMF injection at the same given time (Fig. 3d).²⁹

Based on these findings, operando electrochemical impedance spectroscopy (EIS) further elucidates the reaction behavior at the electrode/electrolyte interface during HMF oxidation (Fig. 3e and f).²⁹ In the absence of HMF, the decrease of the phase angle elucidates that Ni(OH)₂–TPA exhibits faster interface reaction kinetics during the OER process (Fig. S21 and Note S2, ESI†). After introducing HMF, the signal of Ni²⁺ oxidation to Ni³⁺ can be observed in the middle-frequency region (10⁰–10² Hz) at a potential of up to ∼1.35 V, in line with the onset potential for the HMFOR (Fig. 2a). At higher potentials (>1.35 V), signals in the low-frequency region (10⁻¹–10⁰ Hz) refer to the competition of the HMFOR and OER, as reflected by the decreased FEs (Fig. S10, ESI†).³⁰ The overall high phase angle shows hampered HMFOR kinetics at the interface (Fig. S22, ESI†). After introducing TPA, the decreased phase angle indicates enhanced interface reactivity, including electron and proton transfer processes.³¹ Signals in the middle-frequency region reflect favorable HMFOR, bringing high currents and FEs without OER interference.

Hence, in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is applied to monitor the evolution of carboxylate/carboxylic acid under increasing potentials (Fig. 4a). The signal located at 1235 cm⁻¹ is attributed to the C–O–C vibration of the furan ring in HMF, as the enhanced HMF adsorption is the prerequisite for the subsequent oxidation.³² In Fig. 4c, a strong HMF adsorption peak initially emerges at a potential of 1.10 V for Ni(OH)₂–TPA, earlier than the weak peak at around 1.20 V for Ni(OH)₂. The signal of carboxylate (–COO⁻) vibration, associated with FDCA (R–COO⁻K⁺ salt) adsorption during the HMFOR, appears at 1357 cm⁻¹ over 1.40 V for Ni(OH)₂ (Fig. 4b). In contrast, the –COO⁻ signal is present throughout the given potential range for Ni(OH)₂–TPA. This can be attributed to –COO⁻ in the TPA ligand and the overlapped FDCA signal generated in the reaction, as confirmed by the ex situ Raman and FT-IR spectra.³³ However, when the potential reaches 1.35 V where HMF oxidation initiates, a carboxylic acid (–COOH) signal at 1590 cm⁻¹ is distinctly observed for Ni(OH)₂–TPA instead of Ni(OH)₂ as the potential grows.³⁴ This excludes the signal of the adsorbed carboxylic group from FDCA, which usually exists as its potassium salt in KOH once it is formed and desorbed. Conversely, the carboxylate is protonated by the released proton from HMF oxidation, which only appears in the presence of TPA.

Fig. 4 (a) Schematic illustration of the H-cell-based ATR-SEIRAS setup. ATR-SEIRAS under elevated potentials (sampling interval: 50 mV) for the HMFOR of (b) Ni(OH)₂ and (c) Ni(OH)₂–TPA. (d) and (e) Quasi-in situ Raman spectra of Ni(OH)₂ and Ni(OH)₂–TPA for the OER and HMFOR. (f) Performance comparison of Ni(OH)₂–TPA and Ni(OH)₂–BAC with 100 mM HMF.

The same conclusion can also be drawn from the quasi-in situ Raman spectra in Fig. 4d and e. The –COOH peak at 1610 cm⁻¹ is invisible for Ni(OH)₂ both with and without HMF from 1.30 to 1.45 V.³⁴ However, this signal is strong for Ni(OH)₂–TPA, and its intensity is slightly enhanced at higher potentials. This indicates that the free carboxylate is protonated and accelerates proton transfer during the OER and HMFOR. A pair of Raman bands ascribed to Ni^III–O vibration at 479 and 560 cm⁻¹ appears over 1.35 V for both catalysts in KOH, confirming that Ni^III–O act as the catalytic active sites. In the presence of HMF, the Ni^III–O band is still detectable for Ni(OH)₂ (Fig. 4e), while it completely vanishes for Ni(OH)₂–TPA throughout the test. NiOOH reacts rapidly with HMF once it is electroformed, showing enhanced oxidation kinetics. The generated protons during fast HMF oxidation are efficiently ferried through TPA to complete the catalytic cycle. To decipher the role of TPA in HMF oxidation, Ni(OH)₂ coordinated with benzoic acid (BAC) is synthesized in a similar way, denoted as Ni(OH)₂–BAC.³⁵ The XRD results reveal the intercalation of the BAC molecule in the Ni(OH)₂ layer (Fig. S23, ESI†). Ni^III–O signals can also be observed for Ni(OH)₂–BAC in the presence of HMF with elevating potentials, which is similar to that of pure Ni(OH)₂, illustrating its sluggish kinetics toward HMF oxidation (Fig. S24, ESI†). Without the uncoordinated carboxylate (Fig. 4f), the HMFOR activity of Ni(OH)₂–BAC is far inferior to that of Ni(OH)₂–TPA, emphasizing the significance of TPA for proton relay in HMF oxidation. Deuterium kinetic isotope effect (KIE) tests are performed to further reveal the proton transfer behavior in the OER and the HMFOR (Fig. S25, ESI†). Both catalysts show the primary KIE (KIE value >1.5) under these conditions, indicating that proton transfer is likely involved in the rate-determining step (RDS) or directly affects the RDS during catalysis. The lower KIE value of Ni(OH)₂–TPA for the OER and HMFOR specifies that Ni(OH)₂–TPA is less sensitive to the proton transfer process. Thus, shutting the generated proton by the TPA ligand in this step significantly helps accelerate the overall reaction kinetics (Note S3, ESI†).

To explore the application of Ni(OH)₂–TPA under a more industrial-related current density (>200 mA cm⁻²), an AEM-based HMF oxidation electrolyzer is constructed, featuring Ni(OH)₂–TPA as the anode and Ni–Mo/NF as the cathode (Fig. 5a).³⁶ For a 1 cm² (1 × 1) electrolyzer, a current density of 196 mA cm⁻² is achieved for the OER at a cell voltage of 1.80 V in 1.0 M KOH (Fig. S26, ESI†). After introducing 200 mM HMF, current densities of 500 mA cm⁻² and 1000 mA cm⁻² are achieved at 1.74 and 1.90 V, respectively, which greatly outperform those of Ni(OH)₂-based electrolyzers (Fig. S27, ESI†). The preliminary activity of this 1 cm² device demonstrates its potential for scaled-up HMF oxidation applications. Therefore, an AEM-based electrolyzer with a serpentine flow field featuring 25 cm² (5 × 5) is assembled accordingly. The configuration of the electrolyzer is provided in Fig. S28 (ESI†) and the performance is shown in Fig. 5b. Large-scale Ni(OH)₂–TPA and Ni–Mo/NF (25 cm²) are synthesized on Ni foam with a uniform distribution (Fig. S29, ESI†). The device exhibits a current of 3.0 A in 1.0 M KOH at a cell voltage of 1.80 V, and the current reaches 4.0 A at 1.60 V and 10.3 A at 1.80 V by adding 200 mM HMF. The temperature of the end plate and outlet is around 38 °C and 42 °C, respectively (Fig. 5c), highlighting the significance of thermal management in the future design of larger-scale HMFOR devices.⁹ This system runs stably for over 240 hours at a current of 7500 mA (300 mA cm⁻²) (Fig. 5d), making it one of the best devices for HMF oxidation (Fig. S30 and Table S1, ESI†). The initial cell voltage is 1.70 V and remains at 1.68 V after the durability test. Moreover, the morphology and chemical structure of Ni(OH)₂–TPA are also well maintained, demonstrating its superior stability throughout the test (Fig. S31–S34, ESI†). Notably, the electrolyte flow rate is moderately lowered to 3.81 ml min⁻¹ to ensure the complete conversion of HMF (Fig. 5d, inset), thus saving on separation costs.³⁷ FDCA exhibits a low FE at a smaller flow rate due to the competing OER process, while the yield of FDCA would decrease by the incomplete conversion of HMF at a higher flow rate (Fig. S35, ESI†). At the optimized flow rate, the FEs of FDCA are higher than 98.32%, while the yields maintain over 98.33% throughout the test (Fig. S36, ESI†). Based on these results, the productivity of FDCA is calculated to be 2.85 kg m⁻² h⁻¹, which has provided a lab-based paradigm for scale-up applications. By acidifying the electrolyte collected over a 40-hour test, 277 g of the FDCA powder is obtained with an overall yield of 96.1% (Fig. 5d, inset).

Fig. 5 (a) Schematic of the AEM-based HMFOR electrolyzer. (b) LSV plots of the electrolyzer (25 cm²) with and without HMF (electrolyte flow rate: 20 ml min⁻¹ and scan rate: 5 mV s⁻¹). (c) Optical photograph (up) and infrared thermal image (down) of the working electrolyzer at 7500 mA. (d) Stability evaluation of the electrolyzer at 7500 mA (300 mA cm⁻²) in 1.0 M KOH with 200 mM HMF. Insets: chemical flow chart of the HMFOR and 277 g FDCA powder obtained within 40 hours. (e) TEA for the FDCA cost against the current density and renewable electricity cost.

Techno-economic analysis (TEA) further elucidates the merits of electrochemical HMF oxidation modes (Fig. 5d). The HMF cost and renewable energy cost are assumed to be 10k tonne⁻¹ and 0.30 CNY kW h⁻¹, respectively (Section S4, ESI†). The production cost of FDCA depends on the operating current density of the electrolyzer. At 300 mA cm⁻² (this work), the FDCA cost is calculated to be 21.0k CNY with 200 mM HMF, which shows great promise for FDCA production. Meanwhile, the FDCA cost slightly elevates at higher electricity costs, which increased from 20.6k CNY at 0.10 CNY kW h⁻¹ to 21.3k CNY at 0.50 CNY kW h⁻¹ (Fig. S37, ESI†). The concentration of HMF also strongly influences the FDCA cost as the overuse of KOH and subsequent acid–base neutralization are major factors (Note S4, ESI†). In comparison, the FDCA costs are 15.5 k CNY and 32.0k CNY with 400 and 100 mM HMF in 1.0 KOH solution, respectively (Fig. S38, ESI†). High HMF concentration effectively lowers the overall cost (Fig. S39, ESI†). As a result, developing electrocatalytic systems featuring higher HMF concentration with lowered HMF cost is necessary for the valorization of FDCA and downstream products, which needs serious consideration in the future. These results provide a promising approach and deep insights into AEM-based HMF electrooxidation in practical devices.

Conclusions

In summary, we have successfully developed a ligand-modified Ni(OH)₂–TPA catalyst for efficient HMF electrooxidation to FDCA under industrial current densities. Electrochemical tests and in situ spectroscopy measurements reveal that the oxidation kinetics and proton relay process are significantly accelerated in the presence of the TPA ligand. A current density of 850 mA cm⁻² is achieved at 1.55 V vs. RHE in a three-electrode setup, and the FE of FDCA reaches 96.9 ± 0.2% at even 1000 mA cm⁻². An AEM-based HMFOR electrolyzer is constructed for practical application, and a current of 10.3 A is achieved at a cell voltage of 1.80 V. The system operates stably at 7500 mA for over 240 hours, with overall yields and FEs of FDCA exceeding 98.3%. This allows for the production of hectogram-scale FDCA powder with an FDCA productivity of 2.85 kg m⁻² h⁻¹. Our work provides a bio-inspired strategy for catalyst design for HMF oxidation and validates its potential for practical use in industrial-related scenarios, offering a lab-scale paradigm for biomass upgrading through electrochemical methods.

Data availability

The authors confirm that the data generated or analyzed in this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (223B2802, 22409167), the National Key R&D Program of China (2022YFA0911900), and the China Postdoctoral Science Foundation (2023M733178). L. S. is thankful for the start-up packages from Westlake University. We thank the Research Center for Industries of the Future (RCIF), the Instrumentation and Service Center for Physical Science (ISCPS), Molecular Science (ISCMS), and Westlake Center for Micro/Nano Fabrication at Westlake University for the facility support and technical assistant. We acknowledge Dr. Xing Cao for fruitful discussions on this work. We thank Yuqian Sun from ISCMS for NMR data interpretation. We especially thank donors for their selfless help and support to Westlake University.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee05745g

‡ These authors contributed equally to this work.

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